Like Charlie above said, just add more poly.
My method of getting a mirror finish with polyurethane is to apply one coat and let it dry for at least 24 hours as it soaks in and the drying process takes longer. After this first coat I scrape the surface with a single-edge razor blade, watching the gloss on the surface. When the gloss disappears the area is leveled. If, after some scraping, there are still some small areas of gloss left, these are low spots. The low spots need to be filled with more coats of polyurethane, scraping between coats. What you are doing is removing the high spots with scraping and filling up the low spots with polyurethane. When the surface scrapes leaving no gloss spots, the finish is level. At this point you can add one more coat of polyurethane with a brush that has soft bristles like those of an artist's brush and accept the gloss you get with that, or you can rub the surface with OOOO ("four ought") steel wool, the finest there is. After that, wet sand the surface with 600 grit "wet or dry" sandpaper. The object is to remove any lingering scratches. After sanding, use some automotive rubbing compound to rub the finish to a beautiful gloss. A final coat of wax rubbed out helps with the gloss.
Planeman