Three-way switch problem, potentially faulty
If one switch is overriding the other in a three-way circuit, that usually means one of the switches has been hooked up incorrectly.  If you get the common and one of the travellers exchanged, you will see exactly this behavior.

Double-check each switch to make sure you know which contacts are the common and the two selectable terminals, and double-check that the latter two are the ones hooked to the travellers.

(This is not an uncommon error.)