Portforwarding Error


there is a very strange type of problem im facing.I use p2p softwares like utorrent,ares,azureus etc. for downloading.But all the p2p softwares were giving a NAT error that the port is blocked.I had configured my router and forwarded the ports and checked the them from softwares like utorrent and the result was the port is forwarded and recivieng incoming connections,all the errors were fixed and everything was going fine.But after a few days the error was again there that there is a NAT error and the port is blocked i have checked my router the ports were forwarded there but the p2p softwares are still giving erors that port is blocked and in my router settings the ports are forwarded.I have checked all the computers on my network all are giving the same error.I also called airtel service engineer he says that he has no idea about p2p softwares.Now what is the problem i cant undestand because in my router settings everything is ok.I have formatted my pc and againg installed windows but the errors are still there and every p2p software is giving error[Error! Port 11900 does not appear to be open.]now what to do i dont know plz help me,and tell me whats the problem.

my router is beetel 220bx. :(