the quill type stem on my daughters bike, is done up really ,really tight but will still move out of line with the front wheel when pushed hard or knocked.its seriously tight so why wont it stop moving?
help please..

Penetrating fluid required. GT85 or (better) Plus Gas. Good luck!

possibly a bit small? try shimming it with some coke can?

Take it out, clean everything up and then replace. Possibly try applying a liberal layer of threadlock onto it?

I would take it apart, fully clean and de-grease the steerer and wedge, then re-assemble – only grease the part of the wedge that slides as you tighten it, you want friction on the outside… you could also try some assembly compound for carbon bits to give it more bite.

check there is enough thread on the main bolt once you have given it a good clean. if its a cheap bike then it could be a bit short and wont wedge properly when you tighten it.

good idea podge, could well be possible.
have to also confess that when i had it apart i greased it.all of it.
cos they always seize in.maybee thats why. its **** tight mind and still moves.
clean it all out? reassemble with salt water perhaps? 😆