could be time to clean the machine- or time for a needle change- quite often noticing that the stitch quality is not what it should be is a (reminder) it is time to change the needle- the recommendation is after 8 hours of sewing time or with each new project- different fabrics can cause the needle to dull sooner- or last longer- it's all a (fine line) but generally when you notice things are not as they should be --- it's time.