decryption failed: secret key not available

Hello

I`ve got problem with decrypt some files - not all of them but only some part and I get error decryption failed: secret key not available.

Can you help me ? Its look like I lose my secret key ( thats all I can find in web ) but if it so, why I can decrypt some files and some can`t ?

I use Gpg4win 2.1.0 and GPGshell v3.78.