Originally Posted by Jeff Wills

As above, it's likely that you're not applying enough torque to the bolt. It has to be done when new or the crank arm will be damaged by normal riding forces.

They should be torqued to 300 to 400 in-lb.
This is darn near impossible to achieve with a normal hex wrench. I use an 8mm hex socket in a ratchet handle and use both arms to scissor the wrench and crank arm together.

If your bike shop is not using a torque wrench to tighten the replacement crank arms in place, find a different bike shop.