Re-importing Shared PGP Key

 We have a key in PGP desktop that has been shared with an external party. Our organization receives files from the external party and decrypts files. Recently, the key disappeared from my PGP desktop. I imported the key from the original .ASC file provided to the third party but I am getting the following error when trying to decrypt files in the PGP Desktop - Enter Passphrase

1 Unknown keys(s) - It is not possible to decrypt this message because your keyring does not contain usable private key(s) corresponding to any of the above public keys(s)

Another member of my organization is able to use his desktop PGP to successfully decrypt the files (using the same key)

Any assistance appreciated.

Tom Silleck