The lack of bubble sprites is probably the most likely reason.
Xky: Typically, the Keen 4-6 engine caches the graphics for all the sprites you've placed in the level; but there are other sprite images that it won't load this way. An example of this is the "Keen mooning" sprites; if you use the sprite test in most levels, you won't see them, as they aren't cached; but if you edit the Pyramid of Moons in TED5, you'll see an icon at the top-left of a bare posterior. This icon causes the mooning sprites to be cached so that the game doesn't crash when you leave Keen standing on a moon tile.
Obviously in this situation some sprite images are not being cached. I'd suggest trying placing either a schoolfish or a dopefish on the level to see if it fixes the problem.
Basically, caching is when something gets stored in RAM so that it's quickly accessible when it need to be used -- rather than having to load it off the hard disk.
The Keen engine only loads the sprite images that it needs for the level that you're playing.
So it'll cache all the sprites we place in a level, but we should still be wary of using too much at once, right? This is still a DOS game--I doubt it could really hold everything in one level all at once. That's what I was getting at when I said we couldn't have Arachnuts, Bluebirds, and Berkeloids all in the same level. Just wasn't familiar with the technical limits.
Yeah, probably not all.. But biggest monsters with smaller ones are fine I guess. Or really dunno, maybe it's possible with the normal enemy sprites have them all there..