scopesnc thin layer drifted over from time to time
That is bad.
Plate solve depends on finding asterisms (pattern of stars). If a cloud obscures some stars but not others, the asterisms on the plate could be sufficiently modified so you will not find matches to the asterisms that are in the database.
For this reason, make sure there are no chimneys or tree branches that poke into the plate either.
Also, make sure very few stars are saturated. Sufficiently saturated stars are not identified as stars, but large blobs of light, for the same reason nebulas and galaxies are not part of the plate solve database.. As such, the saturated star won't be among those selected, and what is worse is that, those stars are components of the more important asterisms too (brighter asterisms that are likely to be in the database). Focus needs to be tack sharp.
Expose for max ADU that does not go above 65535. A good camera should have enough dynamic range so that there are sufficient dimmer stars. Use camera gains that give the good dynamic range. I would also avoid plate sloving with really bright stars like Sirius or Vega in the frame (and, for sure to keep planets and their moons out of the plate). That is where cheaper cameras may not have enough dynamic range.
100 good stars (none that are thrown away because they are saturated or if they are obstructed by clouds or tree branches) are usually more than sufficient to plate solve. I have succeeded in solving with just two dozen stars.
You can experiment by taking a star field, and editing out some of the stars, and then ask for the modified star field to be plate solved.