There could be a couple of reasons of why I won’t reply to you. One, I could have not receive the letter. This does happen, and even with email as nothing is perfect. The second is you are very vulgar and offensive. In which case, we will not make good penpals. You may also try to convert me to your political or religious views. If I feel that your letters are more like a regular sermon than something more general, then I will likely not reply. I also will not reply to scammers, or SPAMmers. If we had a communication before you will no longer receive letters from me, then I will likely tell you that I will not be writing you any longer. When I do that, any future communications on your part will not be answered.