Every so often a custom keyboard stops appearing. You tap the globe key and it cycles between the default keyboards as though the one you installed was never there. Or it works for a week and then vanishes after an update.
This is a small set of specific causes rather than a mystery, and each has a fix. Here they are in the order worth checking.
First, the setup was probably never finished
Installing the app is one step and enabling the keyboard is another, and iOS does not do the second for you.
Go to Settings, General, Keyboard, Keyboards, then Add New Keyboard. Your installed keyboards appear under a third-party heading. Tap the one you want.
If it is already listed and still not appearing, tap Edit and check it has not been reordered to somewhere you are not looking, because the globe key cycles in list order.
Full Access is the second thing
Many custom keyboards ask for Full Access, and iOS makes this deliberately prominent because it is a genuine permission rather than a formality.
Without it, some features simply do not work: sending images or GIFs, syncing settings, and anything that reaches the internet. The keyboard may still type, which is why people assume it is installed correctly and conclude the feature is broken.
To grant it, go to Settings, General, Keyboard, Keyboards, tap the keyboard, and switch on Allow Full Access. iOS shows a warning when you do, which is worth reading rather than dismissing: with Full Access, the keyboard app can transmit what you type. That is the trade, and it is a reasonable one to decline if you only want the keyboard for typing.
Where custom keyboards are blocked entirely
Not a fault, and the most common source of confusion.
iOS switches back to the system keyboard automatically in secure fields: password entry, passcode screens, credit card number fields, and anywhere an app marks the input as secure. This is deliberate and cannot be turned off.
So a keyboard that "keeps disappearing" is often behaving correctly. Check whether it happens in every app or only in specific fields. If it is only the fields where you are entering something sensitive, that is iOS doing its job.
Updates are the other common cause
A significant iOS update occasionally resets or reorders the enabled keyboard list, and the keyboard app itself needs updating to stay compatible. If a keyboard stopped working around the time the phone updated, check two things before anything else: that it is still listed under Settings, General, Keyboard, Keyboards, and that the app has an update waiting in the App Store. Between them these account for most of the cases where something worked yesterday and does not today, and neither requires reinstalling anything.
When it works in one app and not another
A pattern worth recognising, because it points at the app rather than the keyboard.
Some apps use the system text field, and custom keyboards work normally in those. Some apps draw their own input areas, particularly games, banking apps and anything with a heavily customised interface, and those may not offer a keyboard switch at all.
There is also a per-app memory: iOS remembers which keyboard you last used in each app and returns to it. So a keyboard that seems to have disappeared in one app is often just not selected there, and one tap on the globe key fixes it permanently for that app.
The quick diagnostic: open Notes, which uses the plain system field, and try there. If the keyboard works in Notes and not elsewhere, nothing is broken and the issue is that app. If it does not work in Notes either, go back to the Keyboards list.
Memory pressure, and why it looks random
Custom keyboards are extensions rather than full apps, and iOS gives extensions a much smaller memory allowance than a foreground app. When the system is under pressure, an extension is among the first things unloaded.
In practice that shows up as a keyboard that takes a moment to appear, or falls back to the default one and then works again next time. It is more likely on an older device, and more likely when a demanding app is in the foreground.
Restarting the phone clears it, which is why the generic advice sometimes works even though it explains nothing.
The reinstall sequence, if nothing else worked
Do it in this order, because a partial reinstall leaves the old registration behind.
Remove the keyboard first: Settings, General, Keyboard, Keyboards, Edit, delete it. Then delete the app. Then restart the phone. Then install the app again and re-add the keyboard, granting Full Access if you need the features that require it.
Skipping the restart is the usual reason a reinstall does not help.
Quick answers
Why does the globe key not appear at all?
It only shows when more than one keyboard is enabled. With a single keyboard there is nothing to switch to, so iOS hides it. Adding a second one, including an emoji keyboard, brings it back.
Can I make a custom keyboard the default?
Not in the sense of replacing the system keyboard. iOS remembers the last one you used in each app, so it will come back on its own once you have selected it there.
Does Full Access mean the app reads my passwords?
Custom keyboards are blocked from secure fields entirely, so passwords are typed on the system keyboard regardless. Full Access is about what the keyboard can do with everything else, which is why it is worth granting deliberately.
Why is it slow to open the first time?
The extension has to be loaded into memory. The first appearance after a restart or a long gap is the slowest, and subsequent ones are quicker.
Where to get it
Kawaii Keyboard & Wallpapers walks through the enabling steps in the app rather than leaving you to find them in Settings, which is where most of these problems start.
Check the Keyboards list first. That one screen explains most of what looks like a broken keyboard.