Aspen Dental review: Payment processing

I have been a patient at Aspen Dental in East Peoria for the last year. I went there because my dental insurance said that they covered this dental office and not the office in my home town. I had some fillings and most recently a crown put in. I paid every bill that came to me and even paid a huge payment in person before the work was done. Now I keep getting a bill for the work that was done and saying that my insurance hasn't paid them. After questioning the office staff, I was told that my payment did not show up in their system. I had an authorized letter from my bank showing the cashed check to them. I have called many times since then when I continue to get the bills in the mail. They assure me that they will call me back, but I do not hear from them at all. I am concerned that my credit will suffer from this mess, but don't know how to fix it. I will go into the office personally next week to see what I can do in person. I just want my payment to be acknowledged so that I can move on to another dentist and not have to pay the 900.00 dollar bill along with the 400.00 portion that they say the insurance hasn't yet paid.