(Couldn't find slave-squatter nor heavy-job as components, so posted in core)
I think there's a problem in the cooperation between these components.
I started using them yesterday to replace a more complicated mix of "heavy-slaves".
I have one slave with 8 executors. I configured it, using slave-squatter, to reserve 3 of these during office hours, leaving 5 for use. I then noticed a job with weight 3 claiming to wait for next available executor even though it was the only job in the queue and there were no builds on the slave (or anywhere, actually).
I tried lowering the reservation of the slave to 2 (i.e. 6 executors free) which caused the build to start - but having to keep an eye on these situations is not why I run Hudson
The only issue I could find that smells a bit like this was JENKINS-7033 (Job in build queue is not executed).