App Store purchases not accessible on correct account across devices

For years I've had three apple accounts and purchases from three countries because I've lived in them. Normally they happily coexist on my iphone and ipad, but recently two (the oxford university dictionary and the guardian) stopped working on my iPhone, though my iPad was still fine. Fortunately, both were available in the apple store of the country I currently live in (Germany) so I cancelled both subscriptions as my British me and bought them as my German me. Now they are working fine on my iPhone (where I made the purchase), but now the Guardian doesn't work on my iPad! My login account is set to my German account, but trying to find the problem, I see it says "purchases from [my UK account]." I can understand that Apple might want a password for each of the accounts I have purchases through, but why would it be impossible to access a purchase from the account I'm logged in on?

[Re-Titled by Moderator]

Original Title: Why don't I have access to apple store purchases on iOS on correct account? (Countries / accounts problem again)