So I'm playing co-op with a friend on the Transit level, but every time we get into the ninja-garden bit the game won't let us progress.
 
Once we clear the area of enemies, no new enemies spawn and we can't progress to the right any further.
 
There's no freezing or anything, just a lack of progression. Has this happened to anyone else? It's pretty frustrating really.

Have you tried going around the left and right edges of the screen and just bashing the buttons, preferably while holding a weapon for the extra reach? I've had this happen at certain points and it was always to do with enemies just not entering the screen.

This was the exact problem! We just waited for about 30 minutes and another enemy spawned.
 
hmm.
 
Thanks for the advice!