How do I fix bare spots on my table after applying polyurethane?
Most likely you have spots of contamination such as oil or wax impregnated into the wood. As much as you may not want to hear this, I fear you may need to strip the table top again and sand the surface more completely. When the surface is clean and sanded, you can mist the surface with a little water to see if any areas repel the water. Don't let the water stand on the wood, dry it quickly after your test. This may help you determine if more sanding or degreasing is necessary to remove the contaminated areas.