Sounds like your driller isn't following the pluging and curing procedure properly. either exposing it to too much humidity, or his mixture is off of resin and hardener. He might have air in his pumps as well and cause a poor mix. If he's using the quick curing plug for example less than 2 hours sometimes the quick drying material heat it generates is too much, and causes cracking too.
--------------------
I am Canadian Dude! GOO BIG FIN!!! MIKA K ALL THE WAY!