How to increase the device payment plan amount

I'm about to switch back to my previous provider. I've been trying to increase my device payment plan amount so that I can upgrade our phones that are paid off. I was able to do two of the phones, and when they did the third, it said that I didn't have enough credit and would have to pay out of pocket. Long story short, I've been in and out of the store, and several calls later with customer service, no one knows how to increase it. I have excellent credit; there is no reason for this. Has anyone else had this problem?