The MCX is a very durable heli for its size but can be very delicate in working with the mechanical componets.

You need to be patient and thoughtful how you are going to remove certain parts. The key here is finesse, gently
and carefully. The swashplate little balls can get broken if you pull too hard, to snap off the links use a small
tweezer to push the little ball out from the links, if you use your finger chances are something is going to break,
an average hand can be too rough with these tiny helis so you have to be extra pateint and careful.

Also when working the mechanics you have to becareful not to distort the blade shape this can rob you of lift and it will show up
in the heli wanting to yaw in one direction more than the other. Be careful around those tiny motor wires these can get broken easily if
you pull too hard on something.

CHeck that the main gears are not binding
Check the blade shape again
Retrim your transmitter

if your motors work and the heli is fairly new then it is probably something mechnical or just needs to be trimmed.
Sounds like you already cover the re-trimming prt, so what exacly is it doing that is so terrible and doesn't fly like the first time?