That’s probably an end cap vent clog. If that little valve clogs it causes all kinds of problems like that and it happens early and often with the Venus. Lube gets in there and dries out very quickly. I’ve had it happen in 30 seconds after starting. Then it’s just a constant fight to get it back working correctly again. It’s the one flaw in the system, but it can be easily fixed. You need to take a tooth pick and pop the valve back open from the inside and run warm water through it to flush it out good and clean.
Then make one of these: https://venusformen.com/forums/topic/easy-fix-for-sticky-end-cap-valves/
That should probably take care of it.