There's two reasons this may be happening:

1) The codes were learned incorrectly (assuming not using the code database).

2) The MX-450 simply has difficultly replicating the code format used by that device.

If #1 isn't true, then there's not too much you can do about #2. I've found that some URC remotes can have an issue working with certain modern cable / STB / IPTV boxes.

Sometimes learning the codes "differently" can help reduce sensitivity. It took quite a while for me to get an older MX-900 to play mostly nice with the IPTV box my provider uses (again, high sensitivity issues).