When I use cotton sateen I get skipped stitches with my BSR. I found out it was because the weave of sateen is much different than normal cotton and the sensor can't read it. Check the weave of your fabric. If it's sateen or coated chintz that might be a reason.
Next, try a new needle. Dull needles will sometimes cause skipped stitches. Then make sure your bobbin case tension thingee doesn't have lint stuck underneath it.