Hi:
When you walk through the data connection wizard for your main data source, at the very last screen you'll see at the bottom information about submitting. If there is a reason InfoPath won't enable submit to your table, there will be a message. Here is another post on this subject that may be helpful to you.
Just so you know, the issue is not going to be due to using other data as secondary data sources -- generally, not being able to submit has to do with a data type in the table you wish to submit to. If the other post doesn't help you, attach a copy of your form and I'll walk through the data connection real quick, see if I can see what's wrong.