Skipped stitches can almost always be solved by using a larger needle, especially when quilting. They're caused by needle flex, which happens when pushing/pulling something thick and heavy through the machine. As the needle flexes, it misses the bobbin thread and you skip stitches.
Try a size 18. And before anyone freaks out about the size of the needle holes, size 18 is STANDARD for longarm quilting. If you've ever quilted by check, it's highly likely the needle was a size 18.