Here are some possibilities:
1 Your item list may be full. If you have too many items it won't let you buy any more.
2 (sorry if this sounds rude, I'm not trying to be) You MIGHT have had enough and then bought something else and now don't have enough points
3 (again sorry if I sound rude, but this might happen and yet again I'm not trying to be) you might have counted the zeros wrong.
4 It could be a glitch
5 You might have dropped it and broken the line of pixels where the zero usually would be, thus making it look like only 8 Gotchi Points instead of 80.
Sorry if any of these sound rude, they were not intended to be. I might also have missed something, or your problem may be different than what it sounds like.