I can think of 2 flights I was on where the lavs didn't flush on the ground, but they did in the air . . . .
1) TATL B763. Three toilets wouldn't flush at the gate. They called in a mechanic while we all sat at the gate. He checked EVERYTHING he could, and, I'll never forget his words . . . .
Those toilets WILL flush once you're in the air - - - I'll stake my reputation on it! <Sure enough, they did!
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2) I used a lav at the gate once, and, the toilet didn't flush. A319/320, I think. I commented about it to a FA and was told it would work in the air - - and it did!
Something about pressure. I don't remember, exactly.