JC Penny's Master Cad Are they not cashing your checks just to charge a late fee?" Plano Texas

I have a real problem with my JC penny's Mater Card. I have sent in my payment several times and I receive calls form their credit collections claiming that I have not paid. The first time this happened I figured something happened in the mail so I resent a payment, again I got a call claiming they never got it and they demanded that I make a payment over the phone. The next time that I sent in a payment by mail it happened again, so again I paid by phone. Just now I got a call saying that I never made a payment again, again I had sent the payment by mail, this time I did not make a phone payment, for I believe that there is something very fishy going on here. None of the checks that I sent have been canceled so I know that they are not cashing them or that they are being stolen. But, I am wondering if there is a chance that they are not cashing the checks and sending it to collections so that they can charge late fees. I would not complain if it were just once, but, this is defiantly a pattern, plus they are real rude when you try to explain to them that you did send it and as I stated the first time I sent the payment twice and both times they claimed they never got it.

Am I the only one that is having this problem, are there others? I really don't know what to do on this matter, please advise.
Thank you,

Gsam
Sutter Creek, California
U.S.A.