Ensure the CSC (3 or 4 digit code on the front of the card) is correct.
If the CSC code is correct, check your CSC settings. The following are the setting options:

CSC Settings:
None: All CSC codes are accepted
Light: Y and X CSC codes are accepted
Full: Y CSC codes are accepted

CSC Codes:
Y: CSC matches the code on the record
N: CSC does not match the code on the record
X: CSC was not checked

Refer to What are the Card Security Code (CSC) settings in the merchant account settings? (BB294752) for additional information regarding CSC.