OH that feeling...
You know it is there but won't come out.
I saw this tip many years ago and it works for me
Bend and Raise one knee. Hold it up
Inhale for ten
Exhale and while doing so direct it to your nether region pulling in abdominal area and relax slowly in that position
I talk to my bowels telling then to relax and come on out while doing so.

Repeat
Takes a bit of time but it does a great job
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p.s I have been doing it while writing...now I have to go...........
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