I need help with an old game on my old iPad

I have an old iPad (that I bought) and it still works terrific. I like playing Fishdom 3 Collector's Edition on it, that I bought ages ago. Well, something went wrong with the game and I had to download it again and reinstall. But this time when it asks me if I already purchased it, I clicked on that button and nothing happens. In other times when I had to download and reinstall when I clicked on that button that I already purchased, I was able to continue to play the game. Not this time it seems locked up. Any ideas what to do? Thank you

[Re-titled by Moderator]