What rod have you got??
I use two types of rod for bunnys, the first is an Orvis Power Matrix tip flex (which is not really tippy at all) and one of my own designed rods i build for the Pike Fly Fishing Association which has a similar action, perhaps a tad deeper action.
The reason I dont rate fast action rods for bunnys is because the narrower the loop, the more of a hinge and "clunk" you will get!
Its really important to open the loop more and this will soften out the "clunk". The wider the loop, the less of a clunk you will get.
If you have a fast action rod, increase your casting stroke and this will open the loop more.
I use a minimum of 20 pound mono when using bunnys, and a solid leader with no swivel.
Lastly, retrieve the fly and then left to the surface, do a big circle around your head to get rid of excess water, and then go into the cast. This stops that first "kaboom" on the back cast
I hope that all makes sense....
Rob