SH typically give birth in the wee hours of the AM, so as rayjay mentioned, the fry may be "gone" by the time you see the tank.
also, the male may simply be flirting and displaying his pouch by filling it w/water. look to see is your females are gravid (they will look decidedly fat).
also, are you observing any courting behavior? i know you mentioned that you have a 75 gal tank, but if there is insufficient headroom, the SH may not be able to complete their coital "spins" and egg transfer. this is especially true if you have inexperienced SH. altho they won't last long in your tank, look for clumps of bright orange eggs on the substrate, which would indicate a "missed" transfer.
once you have a known pergnancy, you have about a 2 week gestation period.
and lastly, reidi are one of the two hardest SH fry to rear, so don't get discouraged. you'll likely go thru many broods before finding your "method".