I have the Sapphire 830 and found only coat n clarks works. None of the more expensive threads would work for me either.
You do have to use the green bobbins, dont use any others.
I finally found out how to set the 1/4 inch when I took it in for a cleaning.
The button on the right with the ziz zag symble then arrow to the 1/4 inch.
Then I move the needle to 3.0 to get a scant 1/4 in.