Can you hear the vacuum pump running? This is the klunka-klunka pump not the whirrr of the water pump
Vacu flushes are pretty simple. When you step on the pedal the ball opens and vacuum sucks the waste to the vacuum pump and then pushes it into the holding tank. After you flush, the pump keeps pumping to set-up for the next flush.
No whoosh means no vacuum. So either
the pump isn't running, so no vacuum, or
the vacuum is fine but a clog won't let the vacuum get to the bowl, or
the pumps running but the vacuum isn't building due to
a leak between the pump and the bowl,
a broken pump bellows,
bad or clogged duck bills
or something else
