Game-breaking bug in Endless Zone

Had a weird glitch while playing Endless mode (Wild West/Big Bad Butte to be precise) where after killing every zombie (at least on-screen) for some reason I didn't get the money bag and now I can't clear the level. Maybe there's a zombie left offscreen and there's no way for me to kill it: tried using powerups, plant foods, insta-kill plants and everything. 

Big Bad Butte seems to be basically unplayable for me now, and it seems like there's nothing I can do.