My neice and nephews got drones for christmas, and after replacing blades, none of them will take off. The big one (sky viper s670 stunt drone) seems to have less power on one corner and will try to take off but drag that one motor.

The other two (estes proto z) do the same thing. I was thinking it'd be a simple fix, pull rotors, clean motor shafts of any hair and debres, put together, and set on something flat.. no luck.. try to search for manuals online? No luck.. step 5, ask arfcom..

I notice they all have different  color rotors front / rear.. do these have different weights?

Flip the blade on the dragging motor

It's got a notch in the motor shafts for the set screw to grab, AFAIK it only goes on one way?

You have 2 clockwise and 2 counter clockwise motors, and blades that are matched.  Check that the blade pitch is correct for the rotation of the motor.  You may have mixed up a cw and ccw blade.

Thanks, knew I could count on yas..
S670 is fixed.. now to figure out what props go where on the proto z

We have flight, I repeat.. we have flight.. Thanks alot guys, couldn't have figured it out without yas.

OP, they're paging you in EE.