Besides the problems mentioned above you could be tugging the fabric. It doesn't take much for you to put to much pressure for just a sec which pulls the needle and makes it curve and it does not pick up the bobbin resulting in a long stitch. After FMQ for a time your brain can get set and you start doing that ask me how I now. Make sure you take frequent breaks to get up and move for a minute stretch. So give everything a really good cleaning and then rethread and then watch your speed and hands. Remember to stop when you need to reposition your hands so you do not tug or push the fabric and cause long or skip stitches.