it sounds like the presser foot is sitting to close to the feed dogs, when your doing the doubled hem, look in your manual to see how to take the pressure off presser foot so it a little looser holding the fabric against the feeddogs.

You can try reducing the pressure on your presser foot if it's adjustable on your machine. You can also purchase a walking foot or even feed foot. The feed dogs feed the fabric from the bottom but not the top causing the top fabric to end up longer than the bottom. The walking foot will feed the top fabric evenlyl

are you practicing taut sewing? It can make a difference in long panels. Hold the fabric in front of you and the fabric that has been sewn tautly as you sew it.

A walking foot would almost certainly cure this, I love my walking foot.

Also, I sometimes use wondertape, you can sew through it, it's narrow, water-soluble ( but I've found if it's caught in a seam, it doesn't wash out, never been a problem, but could be for you.) Wondertape is somewhat pricy esp. if you're doing a lot of trim, but when I want to be sure something's in place, I use it. There's another product I've used, not as much for trim, it's called steam a seam, comes in sheets, by the yard, diff. widths, etc. Usually found in quilt notions dept.

Here is a link that might be useful: patternreview.com desc. of taut sewing