Ford Fiesta problems with gears. intermittently cannot select gears.

The car was in a repair shop to have a gear selector oil seal replaced. Since then the gearstick intermittently locks. It was taken back to the repairer who test drove it and " found no fault ". The problem is very intermittent but annoying when it happens. It's my wife's car and I have driven it and not encountered the problem. Today my wife was driving and the gearstick locked again from 2nd to 3rd gear. She said she put it into neutral, wiggled the stick a bit and it then freed up again. I suspect the selector mechanism wasn't replaced properly during the seal replacement but I can't prove that. I'm considering taking it back again to the workshop and insisting the selector mechanism is inspected. Anybody have similar experience?