I had the same problem whilst touring France years back. It was called pyramiding in UK RV circles and was due to me using a macerator too many times, in other words, there wasn't the sudden, big flow you get when you just open the valve into a 4 inch sewer hose. I now try and empty without the macerator every 4/5 times. Glad you got sorted, it's not a pleasant experience to pull the valve and have nothing happen ......
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