@Thommy thanks!
@DefectiveMelon sometimes the colliders get stuck on platforms, the colliders for them are fine I think its just the dodgy way I made them move towards the player means they can walk into the platform and get stuck.
When they die they are still collidable, and have physics so they can fall over, and then after 1.5 seconds it gets disabled. So if it gets disabled midfall sometimes they float.
I might give a go at disabling their physics interactions between each other, and the player, and see how it goes. (..sometime later..) Well I started messing around with it but it made the game perform poorly when lots of enemies started appearing on screen. If too many enemies are on top of one another (drawn on top of each other), fps just starts tanking. This happens faster if I make them more easily to pass in front of each other and they all gather in one spot (like under the platform the player is on).
I could probably do a better solution but it would involve changing the game a lot more than it currently is.