My subs are not in arrears.

I pay my subs every month via EFT and send POP through. But every few months, I get an email stating I owe them 1 or 2 months contributions and will be suspended. I reply with proof of payments attached. But they still tell me I'm in arrears. How can this be? I am not happy at all with this. Can you please contact me to explain what is happening?