PITA but what worked for me (for now) is factory reset, bring all Wi-Fi
points into the same room and set them up again. Then move them back to
where you want them.
I am also having this issue. I have 4 of the current gen nest wifi, and
2 of the old ones. Ironically, the "weak connection" ones are more
commonly the newer ones. I get 250 mbps when connected to the primary
router, and about 20 mbps when connected ...