Hi ash22,
Doing a quick Google search shows that this is not uncommon and not tied specifically to UE4. The following link shows other developers discussing this problem, the last post on the following link in particular sounds very close to what you are experiencing:
iOS GameCenter leaderboard shows only my score in sandbox mode
The following link provides some troubleshooting steps:
Game Center is not showing scores in Leaders-board iPhone
If you feel that the issue you are describing is not related to the links above, please describe how you have set this up in UE4 so we can reproduce the issue on our end.
Thanks,
.