Can anyone help explain why my sewing machine is dropping stitches?

It was sewing fine and then all of a sudden it started just sewing blanks... after I re-did the bobbin actually. I double checked the threading instructions and made sure I had the bobbin threaded correctly but it's still doing it. Every now and again athread will catch so it can look like my threads are 1cm long when they are supposed to be about 2mm.