Despite thinking I had resolved the issue with the updating of My Photo Stream I took a range of photographs and not all were added to My Photo Stream despite using Compatible mode.
I am hoping in the next iOS update that this issue is fixed. 
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I am having exactly the same problem, ever since updating to iOS15.4 on my iPhone12. Camera format set to Most Compatible (JPEG). Just taken four random photos in succession. Two upload, two do not. Weird
I think 15.5 may have fixed the issue.
James, I am on iOS15.5 and can confirm it hasn’t. Taking a series of identical (JPEG format) photos, in quick succession, some upload, some frequently do not. Taking photos out and about, not connected to wireless, when I return to wireless coverage, only a fraction of those photos upload. It is no longer a reliable service, on a nearly new iPhone. I can’t imagine what is interfering with the camera app, on such a random basis. Or perhaps the problem lies with the server.
Long time Apple aficionados often use Photostream to synchronise photos on their devices. iCloud Photo Library works differently.
James, You are right, thanks for the tip. iOS15.5 does now seem to support Photostream correctly, but only when format is now set to High Efficiency. Most Compatible (JPEG) doesn’t work reliably for me. HEIF is still converted to higher compression JPEG, when uploading to Photostream. That has always been the case, and I can live with it.