Valid credit cards saying INVALID?

[Guest]

I do not know what is going on. Most of our credit card payments are going through fine (we do not use real time, we enter the cc here when we ship the order). but 50% of the orders , when I type the cc using the first half , then the middle which is emailed to me, then the last half; when we enter this info, 50% of the credit cards say "invalid" even though our customers say THE NUMBER and exp date is CORRECT. Can someone tell me if they know what is going on? The customer is able to get through and submit the order yet when we try to put it through on our end it comes up invalid.