Reeds can be a challenge. The tale I tell is a wind band rehearsal some years ago - and I had an identical issue playing in a concert a year or so later.Thank you all for your replies. It was better today. The reed was new and maybe need to settle in a bit.
I was also possibly getting in a loop of frustration and tension so who knows.
I set-up to play and we started and nothing came out of the sax - silence. The reed had gone 'limp' and slammed shut and basically wouldn't vibrate. OK. Get a brand new reed out of the box, wet it, put it on. I couldn't get a sound out of it other than some squeaks - it was very stiff like trying to blow a plank. So, I got another new reed out of the box... and it was fine.
So, reed #1 had become very soft and was at 'end of life'. Reed #2 was extremely stiff. I think I did tame it eventually but out of the box it was unplayable.
So, from day-to-day reeds can vary in behaviour - it may not be you.

. Shoulders should be down and relaxed, not raised