Are saying that despite the fabric being placed properly against the 1/4" guide, and sewing straight - that when you measure after ironing the strip is smaller (ie larger seam allowance) than it should be?
If this is the case then you are NOT alone. I've yet to discover a 1/4" guide that is a true 1/4" after ironing. What I do is move my needle position over to the right just a tad. I start by cutting two identically sized scrap strips (say 2" wide), move my needle position to the right a smidge, sew, iron the seam, then measure and the two strips together should now measure 3.5". If not, I run another test and keep doing so until I know where my needle position should be. I keep a post-it-note on my machine with the correct needle position.
If you are saying that you are having difficulty with the fabric pulling to the left in the machine while sewing (the fabric is against the 1/4" guide but is being pulled to the left so by the time the needle hits it, the seam is off) then your feed dogs may be feeding unevenly which could be caused by lint trapped in, on, or under the feed dogs. A good cleaning is in order.