I have found 9 times out of 10 if my machine goes wonky with a stitch and all the tensions are within normal range, that it's the needle. You may not be able to tell if a needle has become bent even slightly unless you remove it, place it flat on a counter and bend down to see it eye level.
I like to sew with a #12, but sometimes even though going through thicker parts of certain fabrics (6+ thicknesses), that needle will bend (#14 doesn't) and then I got a problem.
After a while you get so "one" with your machine, you can almost tell from the purr that something's slightly out of whack. Most always the needle.