Fiber is probably a good idea, but if it doesn't help, ask the doc to check to refer you for a defecogram. It's a test that will show any muscular dysfunction or problems with defecating.
Adding more fiber to your diet should help, but I'm not so sure about the Miralax. This could be caused by a bunch of things, but if the fiber doesn't help you should go back to your doctor and ask for some testing to determine what could be causing this.