Don't understand what you mean by joining forms. I suspect you have a form with two subforms on it, or a form with a subform that has a subform? If so, the relationships might be too complex (many to many to many). If that's not the case, more info about the forms is required. It might be an issue with data that isn't formatted correctly for the joins you are attempting. If the forms are based on queries, try testing each of them to ensure they work independently. Could also try joining 1 to 2 without 3 being joined.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.