Without looking into your data structure itself, just want to make sure that data type for username is same in both sources. Also, it maybe also your custom query where you may have an error. You can blend them (by username) even without custom query unless you have to keep your data structure.
I just wanted to check back with you if solution provided here answered your question or not. If it did, can you please mark as correct answer so that it is cleared from question board that needs to be answered as well as other users in future can benefit from it. If not, please feel free to ask for help. Thank you.