I know, it does seem odd but it could be a fluke that both machines are having the same problem at the same time. Maybe one Machine is having sympathy thread puke.
Did you use the same bobbin in both?
I spent an hour on test strips on my 6600 yesterday, it was skipping stitches on ZZ. I went through many needles(different sizes and types) and threads, along with rewinding bobbins and re threading 10,001 times. I could not get it correct and then it started to miss stitches on SS so I was done. It was for a pair of quick elbow length mittens for my 3yo so I just went ahead and did them. NOT ONE MISSED STITCH!! After I tried on the test fabric, the SAME poly fleece as the mittens, mess and missed stitches everywhere. How can that be explained?