Some of them never seem to stay in place.
You actually need two spanners, as there is one hex that tightens the top race down onto the balls, and another which is a lock nut. And there should be a thin tabbed washer between them which engages with a shallow slot on the steerer tube. Any of those components missing ?
Make sure the LBS tightens the locknut hard whilst holding the race with t'other spanner.
Chances are, if you do it with one spanner, the locknut will either not be tight enough, or it will have dragged the top race round until it's too tight.
Depending on whether your LBS charges ....
these are quite adequate spanners, and will also fit some old fashioned BB cups/lockrings