Repairing bunkers properly is a time consuming process. Quick fixes are available, but don't last long and usually make a mess of the bunkers.
The best method I have found is the following:
1) Clean repair area thoroughly. Best to use a solvent cleaner if the bunker is old and very dirty.
2) Key both surfaces with fine sand paper.
3) Apply a rubber-based glue (the stuff cobblers use to fix shoes is pretty good) to both surfaces, but keep them apart.
4) Allow glue to dry and then reactivate it with a heat gun being careful not to apply too much heat to the bunker itself.
5) Press both surfaces together firmly in a clamp and leave for 24 hours.
Obviously, if you've got a lot of repairs to do, this can take some time. And its still not fool proof - some repairs will hold for ages, others will annoy the hell out of you.