Are you sure you've copied the complete key (private and public). Because the private key is used to decrypt, and the public to encrypt. You can check that by running gpg -K . You can export the private key using gpg --export-secret-key -a uid > file
But I am sure that the keys are correct because after the linux upgradation; we were able to decrypt files for once. But after that day the script's been failing displaying the same error again & again for the reason we couldn't fathom! My supervisor thinks it could be an environment issue. Please suggest.