Quite possibly they aren't BSing you. There are a few threads around here about it. Not exactly sure for 738's but it may be similar.
Originally Posted by
ClipperDelta
They weren't B-S-ing you...differential pressure is what drives the flushing of the toilets on many airplanes. On the ground (or below 16,000 feet for the A320), you dont have enough of a differential pressure so a vacuum generator is used on the A320 to produce the required pressure differential. In this case, it sounds like the vacuum generator was inop, but once you reach 16000 feet (according to Airbus for the A320 family aircraft), the 'normal' differential pressure would be enough to enable the toilet to flush...
http://www.smartcockpit.com/docs/A32..._and_Waste.pdf
https://www.flyertalk.com/forum/delt...-000-feet.html