When you run a piece of fabric through your sewing machine, you use a sewing needle. This needle can and probably will pierce the fabric in just the wrong place. This will cause your seam to split or pop and the fabric might even be ruined. So how do you avoid this and still use knitted fabrics in your projects?
To avoid skipped stitches and popped seams, switch to a ballpoint needle. You can get these at any sewing supply shop and most craft stores. They're often labelled as being "for knits" so they're not hard to find. Ballpoint needles have a rounded tip and are designed to slip between the knit fabric loops instead of piercing the fabric itself. You should also consider using a polyester thread when working with knits. Polyester thread is both stronger and more flexible than cotton, so it will stretch with the fabric instead of snapping as cotton threads might.
So before you start working with knits take the time to get the right supplies. It wills save you the grief and aggravation of a ruined project or two.