Help with Brother PE770 - Upper thread looping on bottom, no bobbin thread showing
 

My wife and I have had a Brother PE770 for a year or so and have been able to resolve most issues by running down the checklist (e.g. rethread, reset bobbin, clean bobbin area, etc), but this one has us stumped. All of a sudden the upper thread is stitching out very loose, with big open loose loops on the bottom and virtually no bobbin thread on the bottom at all.
We've made no changes in how we embroider at all - standard upper and bobbin thread we always use, same fabric, same fonts, etc. This just started happening out of nowhere. It even "fixed" itself at one point in the middle of a pattern where it had been stitching out loose, only to go back to stitching loosely.
We've seen some posts online saying the upper thread tension is loose and we should increase the tension through the digital settings, but (i) we've tried that and it didn't work and (ii) we've never fiddled with those settings so it shouldn't all of a sudden need that digital adjustment.
We are a bit desperate and would love any help or tips!

Check your bobbin assembly. I mean take it apart like you would do in a cleaning session. Now get a magnifying glass and inspect the metal blade where the thread goes through. Check for any lint even the tiniest bit can mess up the stitching. My PE770 was messing up like yours and I found a tiny speck of lint stuck between the metal and the plastic. When I removed it my machine ran perfectly.

It may be the design. Have you stitched it previously to know that it stitches without issues?

Are you using a pre-wound or your own bobbin? Make sure the bobbin is not squishy if you wound on the machine.

Change the needle.

Clean out bobbin area.

Check the thread cutter area for stray threads.

Does the machine stitch regular sewing stitches OK or is there issues too?