It may be the timing, or something else, but the latter doesn't seem to make sense in your case. if you hold your breath, or even do a Valsalva maneuver, it should work: Take a very deep breath, Push that breath out against your closed mouth and nose while straining as if you’re trying to poop. Hold that for 15 to 20 seconds you should have lower blood pressure during those 15 seconds. So, time this maneuver to coincide with the cuff inflation such that you are in Valsalva slightly before the cuff has reached its pressure max. There will also be a compensatory momentary overshoot of blood pressure directly after the manoever. If a person's blood pressure neither drops nor rises in compensation after the maneuver, that's an indication of heart failure. But I don't think this applies to you.