How should I handle areas of soft polyurethane left after sanding?
Has the polyurethane in areas other than the gummy spots dried alright? If so, then maybe these spots are the result of not stirring the polyurethane enough prior to application. Sometimes the the product settles in it's container so a couple of slow and steady stirs is needed. Second possible cause could be application method. How was the polyurethane applied? Assuming your able to access all spots and that all areas around the spots did dry properly, I would take a rag dampened with some mineral spirits to those spots. Try taking them down to a level that matches the surrounding areas.