When sewing fleece I use my Singer Featherweight which sews without any issues. I have a Janome which always makes so much noise when going over a thick seam.

Make sure you are using the right sized needle, use a walking foot if you have one and if needed loosen the tension a bit. Rarely would timing be an issue if it was sewing fine before the thick fleece

Clean out the fuzz in the bobbin case! I sew a lot of fleece blankets (double thickness) and it will pick up fuzz quickly. I use an 80 needle, walking foot( let it pull fabric thru , that is what it is for) and a zig zag is what i use as it gives stretch on the blanket and length the stitch length slightly! Clean out the fuzz in the bobbin case if most important, try that first. Good luck. P.S. I make fleece blankets for homeless people and I always have to clean out the bobbin area!!!!

I made a fleece hat the other day and did really well using my walking foot. It kept it from slipping.