Charles, there are two options, one is conform to  and the other is in the Object drop down menu >Change parent--change it so the shorts are parented to the hips and see.

It might be off in frame 30, if so, move the shorts there and see if they stick when you move or adjust your figure. 

Maybe part of the boxers is conforming the rest is dynamic, check in the Cloth room and see if the shorts come up under clothify.  If it's dynamic, it will show up there.

Also some clothes are sort of fuzzy, you will need to place them exactly on the figure in the zero mode. Both for conforming and dynamic.  Jan