My guess is that you are using an older, "quill
" stem, which is both the STEM
(the extension that the handle bars attach to) and the friction bolt that tightens the stem into the STEERING TUBE
Anyway, what I think has happened is that the quill is either not tightened or (because you said you tightened everything) the angled nut at the end of the quill isn't aligned properly and is tightening. HERE
is a great diagram of the quill stem. What I think you'll need to do is:
1 ) IF you have already tightened the bolt at the top of the stem (usually under a little plastic cap, like the one at the top left of the stem in the diagram). Then Untighten it and if you can pull it out make sure that the angled nut (shown at the bottom) is resting properly against the angled shaft of the quill (the vertical part of the stem). As you can see these will spread apart as you tighten the bolt, making them wider and holding them in the steering tube. The idea is to realign the angled nut so it can properly contact the shaft and spread.
2) if you haven't tightened this bolt (maybe because it's hidden), try tightening it a small amount and see if that changes anything.
This was all written assuming your bike is not using the newer "threadless" setup, which would probably have been fixed if you tightened down the stem (PICTURE OF THREADLESS STEM