If sewing quality starts to deteriorate, you may need to change your needle.
Choose a size appropriate for the thread you are using:
#65 or #70 (9 or 10) for #50 weight thread
#75 (11) or #80 (12) for #40 weight rayon and/or #40 weight polyester
#80 or #90 (12, 14) for metallics or #30 weight thread
Use a ball point for knits, and a sharp point for wovens.
A good, general purpose needle for many applications is a #75 light ballpoint needle.