The base units for the Vernier Dynamics System tracks attach by sliding a nut into the slot on the underside of the track. Some units were shipped with nuts that were a little too thick to slide easily into the slot. This is due to a manufacturing variation in the nut thickness.

If this happens, Vernier will ship replacement nuts.

VDS tracks shipped after October 2007 should not have this problem.

Related:
What are the dimensions for the different slots on a Vernier dynamics track?