You're problem is not unlike many Bass boat guys.
Hauling ass and getting good depth readings usually don't go hand in hand.
This is a very common complaint with high speed boats.
You said that your transducers were in the back. Does that mean on your transom?
If so, then you may be going so fast that your transom is lifted up out of the water so much
that your tranducer(s) are no longer submerged. If the transducer is not submerged, it
can't possibly send and receive pings through the water.
You might need to put one transducer thru your hull, forward of the transom so that
it is making good water contact under high speed. Or possibly a shoot through the hull
transducer, which is also forward of the transom, but shoots down through the fiberglass.
Greg would be able to help you decide which is best.