I often need to leave the morph brush (select another tool) and reselect the part if the morph brush accidently grabs the wrong one, then go back to the morph brush.

I also note that you should select an object part and not select the whole character by going to "body", or the brush won't work. Sometime the brush will push instead of pull on parts near edges and vice versa.

The brush also slows to a crawl if used in complex scenes so I try to do all my morphing before I inport. This could be your issue.

It has been suggested that in the next version (or SR) of Poser you should be able to "lock" the brush to a part so it doesn't wander off on it's own.

I hope this helps you.