Welcome to the boards. It's so great to have new members & I hope you will stick around after your question is answered & show us what you are making.
This manual might help you. Page 14 of the manual shows the stitch length dial. Maybe all you need to do is change the length but it sounds like it may be something different. Page 37 of the manual talks about the machine skipping stitches. This may help you.
http://www.pfaff.com/SiteMedia/PFAFF...-manual-EN.pdf