I dont think it is ideal. If the head slips then the vacuum chamber will be pulling on foreskin, isnt that right? Having the
sleeve pulling on the shaft skin is still distributing shear forces and stresses, however I do think that most force should be concentrated on the glans. My glans doesnt slip, even after 10 hours tonight. Still I remember times when I lost vacuum quickly, mostly within the first minutes after setting up the
Phallosan. Personally I use larger vacuum chambers and suck it up until the glans has expanded - interestingly enough it wont shrink afterwards since the vacuum stays intact (this is a higher vacuum coming from the
Mityvac pump).
Is the Phallo still effective when the glans slipped? Technically if you sense there is still tension on the shaft then it should be working. But the efficiency is definetly not at its maximum. For beginners however I think its good that these things happen because as I remember in the early days it happened quickly that the glans took on white surface color or the foreskin was bloated due to fluid gathering there.
But the reason why all of this is happening is most likely a suction issue. Either you have a tiny hole in the
sleeve (always careful when rolling the thing) or the combination protection
cap, chamber hasnt been worked out yet. I would continue experimenting a bit, usually it just needs some experience to find the error.