The fact that you are missing partial credit suggests that there is some issue with your MP#.

1. Take a look at your e-ticket receipt (not confirmation, but actual receipt). In the various disclosures at the bottom, you will see your baggage allowance for the ticket in question. If that correctly shows your allowance, simply send an email request to UA, wait 5 business days and then initiate a chargeback for the bag fees (unles, of course, you see the credit). In the chargeback, be certain to note that the e-ticket receipt specifies an "X" bag allowance. If your CC issuer permits it, attach a PDF of the e-ticket receipt with an arrow pointing to the relevant section (nobody at an issuer is going to read through 3 pages of stuff that doesn't matter).

2. If your e-ticket receipt does not show the baggage allowance, you have a more serious problem and a chargeback is not going to solve the problem. Call UA and ask them what happened. This may take some time to be resolved.