Rumble Server
You can get missing bots here, thanks to Rednaxela: [1] - we should link this somewhere.
I'd try deleting robots/robot.database (and maybe robots/.data) so that it rebuilds the cache and see if that fixes the missing bot issue. I've been copying my RoboResearch dir to a RamDisk lately and seems like sometimes I have to rebuild if I run from a new location.
I run 1 instance per core in tests, sometimes with a slightly increased CPU constant, but for the RoboRumble, I think the consensus is to err on the side of caution and 1 run less than the number of cores, since Java / Robocode / operating system incur some overhead.
Several times when a particular rumble directory has seemed to get spotty on me, I've opened the actual robocode client in that directory, had it clean the robot cache and then shut it down. This has worked like a charm every time, since (I think) it deletes the relevant directories and fully rebuilds the robot database from the available .jar files. Cleans up the place, as it were.