Problems with Megs smooth surface clay

Tried this clay out on a family members car and was very dissapointed. The clay seemed very flimsy and broke apart during the claying. I am admittingly a total noob at claying and detailing in general, but have seen people clay thier car and never seemed to have a problem. I used about a quarter of one of the two clay bars that came in the package. I used nxt wash as lube. Any ideas on what i should do different or is Meg`s clay not very durabe? All help is appreciated.

Hmm. I know salvents can break down the clay. Perhaps the wash was the problem, though I`ve used dish soap and never had a problem.
Was there tar on the car?