Some builds like oe-selftest require specific workers. These should be assigned first before builds which can work on any worker.
Also, it seems if the next build requires a specific worker, there are times this seems to block any further builds being run until one is available.
Patch is on yocto-autobuilder2 master: http://git.yoctoproject.org/cgit.cgi/yocto-autobuilder2/commit/?id=b5a047f99f60595372aaeda9de9400801c83e7f2 Will be tested after the controller is restarted tomorrow.
Marking this as done since it's been in use for a little while now (with the addition of some extra patches from Richard).