I’ve run into this exact thing and it’s one of those issues that feels mechanical but usually isn’t a broken vacuum. The clue is what you said, it climbs partway, then just sort of gives up and slides back down. If it was dead or faulty, it wouldn’t even try.
In my case, the first limiter was flow. The filter bag wasn’t totally jammed, but it was dirty enough that once the cleaner was fighting gravity on the wall, suction dropped just enough to stall it. On the floor it looked fine, which is why it’s misleading. After cleaning the bag and making sure the valve was sending enough flow to the cleaner, it already started climbing higher.
The second piece was traction. Even if the walls look clean, a thin film of oils or fine dust can make them slick. I couldn’t really see it until I brushed the walls and felt how slippery they were. After a good wall brushing and cleaning, the vacuum suddenly had something to grab onto and started making it much higher instead of sliding back.
If it’s climbing a little better after cleaning the bag, that’s a good sign. Dial in the flow so it’s in the recommended range, make sure the water level is high enough, and give the walls a proper brush. Most of the time it’s a combo of slightly reduced suction plus slick walls, not a bad vacuum. Once both are sorted, they usually start running all the way up again like nothing was ever wrong.