if your loops are on the bottom it means your top tension is too loose- you may need to tighten it a bit to make up for the different materials. adjust it just a little at a time- like a 1/4 turn- test- then tighten a little more until you have the top & bottom even--if the top stitching was not right it would mean the bobbin tension needs to be adjusted-but it sounds as if this is not the case-