Up until recently I used to be able to get a crawler by using explosives and gain some time to rebuild barricades and get weapons in between rounds. Now I can't seem to get ANY crawlers in Solo and I have tried many times with grenades, rocket launchers, etc. into large groups of enemies on levels as high as round 80 with no crawlers produced. However, I can play multi-player and get crawlers fine. Any ideas?
Apparently, this is a not-too-common but known issue. Crawlers are blocked when the Graphic Content settings of the game are set too low. With graphic content set low, there is no red blood allowed, and no "missing-limbs" enemies, which would explain the no-crawler issue.
To fix it, start the game, then go to Settings > Game > Graphic Content and change it from "Reduced" to "Unrestricted".