Q: Purchased apps missing from my account

You've checked to see if the app is hidden from the Purchased tab in the App Store app (the hidden section will only show on your account if you have hidden items) : Hide and unhide purchases in iTunes or iBooks on your Mac or PC - Apple Support ?

If the developer has removed the version that you bought and released a new app instead, or potentially if it's changed developer, then it won't show for redownloading. If that is the case then do you have a copy of it in the computer's iTunes library that the iPad is synced to so that it can be synced to it ?

Try this:

1: Launch the Settings app, scroll down and tap on iTunes & App Store.

2: Tap on your Apple ID, and select the ‘Sign Out’ option from the pop-up menu.

3: Type the password to your Apple ID to login again.

Check if you see purchased apps.

If this does not work: Sign out and sign in to iCloud.

You can sign out and sign into iCloud in Settings → iCloud

Purchases may be also hidden. Check if you can unhide them:

Hide and unhide purchases in iTunes or iBooks on your Mac or PC - Apple Support