A few ideas:
Your watch might need replaced. With my old Series 5, I always got smooth sinus rhythm readings, every time.
The ECG on my Series 6, after a few months of owning it, stopped working. It would constantly go back to 30 seconds after a few seconds of reading, and was often inconclusive.
My Series 7? Flawless, never a bad reading.
Stuff like wearing your watch too high on your wrist, too loose or too tight, the back of the watch being dirty…it’s likely that none of these are the problem here if you’re always getting inconclusive readings.
I’d take it into the Apple Store and show them. You might get a new (refurb) Series 5 (or maybe better) out of it.
Then you’ll have an Apple Watch with a functional ECG and a brand new battery. That’s a win-win.