Product: Credit card or prepaid card

This is regarding synchrony bank ( holder of the store card for Amazon.com ) : First it will help you to know how I use this card and why I think what happened doesn't make sense. I have an amazon card ( through synchrony bank ) I use it primarily for the promotional purchase ( deferred interest ) quality and I frequently put items in my cart and wait for my purchase value to hit one of the promotional value so that I can obtain differed interest- I am retentive about when my promotions are coming due and I try/succeed in paying off my values each time ( or so I thought ), I make 2-6 payments on a average month so I am in my account all the time. Sometimes I make a one off " urgent purchase '' but I just adjust the amount I had planned to pay in order to pay everything off as to not accrue interest b/c I was doing some requested research by my financial adviser I opened my bill and was surprised to be greeted with an amount that was accruing interest.. There is a $ XXXX amount accruing interest ... 'WOAH, what?! from where/when? ' So, I reviewed every bill I have access to & I can't see why this is happening but I notice something else as well - the amount that is accruing interest seems to be going UP. 

So last night at XXXX PST on XX/XX/19 I call Synchony Bank and I ask them ... 'So, I see that I am accruing interest on my account and if I owe it I am glad to pay it but can you help me understand what is causing this to happen? ' I explained how I pay a crazy amount each month to manage/maintain my card so that I can continue shopping when I need/want with no interest and how/why I utilize this card.I was on the phone with them for more that 2 hours and at the end of the call that could not definitively advise me WHEN I made the purchase that resulted in interest ... so at the end of the call after he tried to get me to at least 2 managers that would not talk to me the resolution was basically to " send ME all the bills and let ME figure it out. Several times during the course of the call I requested/recommended that they refer my account to an in house audit group to check when the interest baring purchases took place and verify that interest is applying accurately to my account : that did not happen. 

So at the end of my call I now have TWO major concerns : The first is based on my understanding of the 'consumer protection act ' after a two plus hour phone conversation they could not definitively prove to me/show me or even tell me when/where I started incurring these interest baring charges and it is my understanding that if a debt holder can not tell you/prove to you PROVE to a court or otherwise when/where HOW you incurred an interest baring debt then it is a noncollectable debt. 

But in calling about this I discovered a bigger potential problem ... besides not being able to prove to me that the debt was owed my BIGGER concern is that I don't think that the payments are being applied to the accounts correctly. What lead me to this belief/discovery as I was looking at my bills over the last several months dispute paying several and repeated payments the amount that is baring interest actually is going UP. And I really don't understand how THAT is happening either and through no fault of there own the gent I was speaking with could also not explain why or how that was happening.