LookAhead hang with depth 10

My AVC encode always hangs when using LookAhead BRC with depth 10, but not other values (11, 40, 100). I have permuted a few things like source video (including different sizes), target usage, target bitrate, and they all hang. It looks like 10 is supposed to be a legal value.

Is there some other interacting parameter that I may have set incorrectly?

Thanks,

Ryan