The parameter you may want to check is collision distance in the prop's collision settings.

Default value is 0.5 cm. You may want to reduce to 0.2 or less. Poser uses this parameter to determine whether a vertex of the simulated object has collided with the collision geometry. It is normally used to prevent poke-thru and falll-thru and you can use it to simulate cloth thickness or clothing layers underneath.

Alternatively put the vertices of the waistband in the 'constrained' group. They will then not be pushed away from the figure to the collision distance.