Thank you for your feedback! The only way we could imagine this happened was if there was too much TNT and all the materials got destroyed. It is possible to use the same blocks and place them back and forth to progress when materials are sparse.
We are also currently discussing on how to best optimize the pacing issue with BOB as in a normal case BOB will move as fast as the camera does but can generally be stopped by other materials to fall behind. You then can use materials directly placed behind and in front of him to give BOB a little push!