If you insert the stem too far it may be trying to tighten on the tapered lower part of the fork steer tube which is normally internally butted. Try inserting it less, though to or below the minimum insertion mark, and see if that helps.

Otherwise you might have split the steerer tube at the groove that is machined into threaded steering tubes. Seen photos of it happening with aftermarket forks which are threaded and grooved too far down in an attempt to allow fitting to a wide variety of frame sizes. I would remove the fork and check the steerer for damage.

If the steerer tube is swelled or split then you will need a new fork.