Serging Fleece Blankets

Hi All! I was just given an old 1989 Singer 14U85 Ultralock machine that has not been used for 20 years and it works fabulous! I am a very new beginner and have been watching all kinds of YouTube videos and will be taking classes soon. I am practicing making double sided fleece blankets and have noticed that the stitches are perfect when I am serging the stretchy side of the fleece but when I sew across the edge that is not stretchy the stitches will skip when I pause and go. (I think there is a term for this when fabric will stretch one way but not the other?) When I am sewing the sides that don't have that stretch to the fabric do I need to change the tension? I am using the basic 3-Thread Overlock stitch. Thanks for any help!