Following the above advice should eliminate contamination as the cause of the problem. If the problem persists, could it be that the finish is not receiving a complete scuffing? Flat sanding sticks are great for flattening 'nibs' and leveling high spots, but may not be covering the entire surface. synthetic steel wool (000), I find, works well for scuffing all of the surfaces. It can reach under guides and flow over wraps. It can even soften the sharp vertices, helping the finish to flow over them. Since polyurethane finishes have been notoriously poor at bonding to themselves, it is very important to give an aged surface a complete scuffing.
Something else to consider is that since these finishes cure from the surface inward, scuffing the surface exposes semi or un-cured finish to the atmosphere. This should be good for adhesion of a second coat. However, If the entire film was not fully cured at the time of scuffing and then was left for a few days before re-coating, that scuffed surface would then become a 'new' cured surface with the same limitations of adhesion and need for scuffing It would seem prudent, then. to re-coat shortly after any scuffing.