Denied Claim - procedure code modifier invalid

I have several rejected Blue Benefit (BCBS) claims for two clients, which indicate:

"Acknowledgement/Returned as unprocessable claim-The claim/encounter has been rejected and has not been entered into the adjudication system. Missing or invalid information. R SERVICE LINE PROCEDURE CODE MODIFIER INVALID; MUST BE A VALID CMS (HCPCS) MODIFIER [02] (A3 - 21)"

I've been given the run around with BCBS.  Does anyone know how to resolve this?  Thanks!