OK investigation complete!
The hits are actually connecting they will just be doing massively reduced damage. This enemy has a set max damage in any one hit so any single piece of damage coming in will be reduced to 10. Only a handful of enemies in the game have this (for queens its 25, this rove beetle larvae is 70 and then this is 10 so this creature actually has the best). As such it will be terrible at killing it with a health of 550, each bit will be doing just over 1% damage. The counter is of course to use either venom strike which does multiple small hits or build something like a rapid fire tower.