When I re-did mine(after the head job set), I hit all of the areas where the previous leaks were comming from, and still have a small leak. My Sister had a leak on the passenger side cover show up shortly after the dealer adjusted the valves, so simply following the FSM doesn't always do the trick. When I do them again I am going to hit the entire outer edge w/FIPG.

A crack could be the cause, but unlikely unless you over tightened a bolt when putting them back on. An inch pound torque wrench should be used over a Ft lb or hand tightening w/1/4" drive and not a lot of force.