I have a feeling there's a micro leak in the sleeve because of the sleeve's folding or rolling. Try to move the sleeve up a bit more, covering at least 3cm over the vacuum cup. Near the top of the vacuum cup, seal the sleeve's opening with a wide band of somekind, such as a cut out of a smaller sleeve, rubber band, or even just your hand, if you have one to spare during pumping. The seal for your skin should be nominal without leakage. But I can be wrong. Worst case, either send me a photo privately to evaluate if you're shy, or post it here.