I have same issue, traced all lines both vacumn and electrical and everything appears to function as planned, but still have problem.
Try this, I noticed mine work better when cold start in the morning. Vacumn lines are closed tighter than warm? I dont know.
Next time try revving up the engine 2000 rpm, then as revs fall press the headrest button.
Mine drop every time now. After spring comes I'll take a closer look as to where the leak is.
71 250 Coupe
92 400E Sedan
90 944/S2 Cabriolet