Originally Posted by barba

I usually don't post in BMX, but you want to get that crank bolt down real tight. Slip a pipe over the wrench and really give it a good tug. Riding a loose crank will deform the crank arms because they are soft AL and the BB spindle is hardened steel. This will ruin just about any crank fast and coming loose frequently is a good sign damage may have happened.

If it is a new bike, the shop may have screwed this up by under torquing the crank bolts. I would inspect the crank carefully and ask them to make it right if there is damage.