Are you getting the same results all the time, under the same conditions?
I haven't read all of the replies ... so sorry, if this duplicates.
I noticed several pointing out that it could be cutting and pressing. Another suggestion would be to move your needle, assuming your machine lets you!
With my machine, the 1/4" foot does not give a 1/4" seam with the needle in the default position. Once I move it over a little bit, bingo! I've got a perfect 1/4" seam ... providing I use the guide on the foot.