Many of the leves where this happens, the mob is running away to a new location to spawn more enemies. This is part of the leve. If you kill the mob running away, you should still be able to complete the leve. If you've done the leve before and remember where the monster intended to run to, you can go to that location and the new mobs will spawn. However, if you have no idea where to go, I don't think the location is marked on the map. If that's true then this would be a nice feature to add.