I bought 2 flywoo mini quads (cinerace 20 pro and firefly baby quad hd) and the 2 hit the ground pretty strong and now in each, there's a motor that won't start spinning. It tries to spin and then won't. They don't seem to be stuck, I think the shaft is ok. On one quad the motor seems a bit stronger and on the other quad it's not strong to turn any of them. I disassembled both and couldn't observe anything wrong in the shaft or magnets, and also they were not burnt.

Is there something that can be done to fix them?

I am trying to interpret the use of your words "strong". Do you mean notchy (a touch of slight resistance to rotation)? Some motors depending on the strength of magnets used in the design can be smoother while others notchy.

If the bearings and shaft appear to move freely then its probably not a mechanical issue but an electrical one. The MOSFET's on the ESC can be damaged (common even in a crash and a crash is a crash and everyone scales of light to hard crash is different).

It could also be the motor windings which can be testing using a hand drill to rotate the motor and measure the voltage at the motor wires which will indicate if any of the windings are bad.

I think best test is to swap bad motors with a good one to rule out motors being the issue.