There's an issue currently that affects all sub robots of BoardRobotGenericBlockBreak, where when they are looking for the next block to break, if there is no block matching their specified target close by, the robot just stands there. Even if the block suddenly appears near by, for instance a tree grows. The affected robots are the harvester, lumberjack, leave cutter, miner and shovelman.

This issue is caused because the block searching logic gets stuck looking for the block in distant places taking a very long time before ending a cycle and starting again. This only happens if the robot has access to open sky, so if you trap the robot in a sealed area it doesn't happen.

Currently the block searching is being done with the pathfinding algorithm, wich, as SpaceToad mentions in the code https://github.com/BuildCraft/BuildCraft/blob/6.3.x/common/buildcraft/core/utils/PathFinding.java#L72-L74 , is probably not the most efficient. Maybe using a better algorithm might fix it, I'll try and report back. Mainly making this issue for users seeing this behaviour.

I'm glad someone is looking into robots! I never really had enough time, as there was just too much to do.

Note: Bug fixes of this type go into 6.2.x, while new features go into 6.3.x.