Could be several things wrong.
Here are the two most likely.
1. Not enough castor as the shop has told you. I race cars in a straight line and want a bunch of castor put into my font end to keep the car going straight.
2. Worn steering box. Play in the box can be a nightmare. You did not say how many miles on this unit. Even then it does not matter much. There are good boxed out there and bad boxes out there.
There are several more but the two above are the most important. If you get the shop to do a castor adjustment get a before setting to see how far they changed it..