Hi Kec00013 and welcome,
I'm guessing you are creating a list form. May I suggest creating a full-fledged InfoPath form library form? It will be less restricted.
It's probably that your issue is related to your list column being a lookup on a different list. If so, please try to add a separate column that is not a lookup. The other option would be to connect to both lists and use the ID from the lookup to filter the other list.
Anyway, I'm guessing it's because there's a limit on the list form that is causing this.
Just a guess though ... :)