Brian beat me to the punch, but here are a few questions, for some clarification:
1) Do ALL of the remotes work with the TM751?
2) Are you trying ALL of the remotes in the same location?
3) Are you trying the exact same commands with all the remotes?
4) Are you testing the TM751 in the same outlet as the CM15A?
5) How far away from the CM15A are you trying the remotes?
6) Do all of the remotes have fresh batteries? Have you tried swapping batteries from one of the "bad" remotes into a "good" remote and testing again?
As Brian stated, the CM15A is known to have much more limited RF reception range than the TM751 or the RR501 transceivers.
There are mods you can do (both invasive and non-invasive) to boost your RF reception range.
If all of the remotes work fine using the TM751, but only some are being picked up by the CM15A under identical test conditions (same codes, same location, same batteries), then you may have bad remotes.
However, I'd venture to guess there are other variables at play here (the CM15A's limited RF range being the biggest), and you might be able to solve the issue by trying to fix the CM15A first.
There are a number of threads here discussing the various modifications that you can do to improve your CM15A's RF range. The simplest involves taping a wire to the outside of the unit (no soldering involved - and does not void the warranty). The most complicated involves opening up the unit, soldering in a connector, and then building an external antenna. There are some solutions in between those two, as well.
A quick search of the forums should give you plenty of information on the subject.