To clarify, you have an a file that was encrypted with a key pair elsewhere. You have been provided that trading partners public key and have imported that to your key ring. You are then trying to decrypt this file and it is stating that no matching keys existing in your keyring.
Please clarify if this is correct.
The most likely cause is that either the public key you received was not the one used to encrypt the file, or you are pointing to a key ring that does not contain the correct public key for decrypting.
Thanks - Jon