Plugin changes do not apply

Hi,
I have wrote a payment plugin for NopCommerce 2.30 and it is working, but when I change its code and rebuild it (and it makes dll files) and copy and replace the new dlls to old dll files, these changes do not apply and the plugin works like previous version (before changes).
I used some tricks: change global.asax, change web.config, restart application (from admin panel), clear cache (from admin panel), Reload plugins (from admin panel)
but it didn't work.

How can I fix this problem?

Thanks!

1. Could you please try using a Reflector in order to ensure that these DLLs are really new?
2. Are you running in medium trust? If yes, then manually delete all DLLs from \Plugins\bin directory

Thanks for your answer

1. Yes, I checked the dll by reflector and its the new one.
2. No, the application is running in full trust.

another note is that when I rebuild the entire solution (in local computer), the changes are applied and the new version works. (that means some main dll files are important and sould be touched to reload my plugin)