Steve,

I was getting this same error, and it seems to be a generic thing. There is actually more specific error information available but for some reason it doesn't get printed (maybe it gets stored somewhere, but I couldn't find out where).

To access it, you can modify line 178 of uc_virtualmerchant.module from this:
'message' => t('Credit card payment declined wHAT.'),

To this:
'message' => t('Credit card payment declined wHAT. ').print_r($response, TRUE),

This should give you the specific error code and description of the error that you are getting.