I had that happend to me, so I further researched, and found out that you have to jump farther out...

run and then jump (a normal jump distance, not like hitting the wall and coming down, then hitting crouch....need the full jump! this will keep the zombies from coming to you

also, here is another catch....when you do the glitch, then you need to have one zombie left for what ever level you are on only because what ever level you are on the glitch starts on the following level....so if you do it in the middle of the level, then they still come...

hope this helps

FSTMRFR on PSN