Drywall stain bleeding thru after 5 coats of paint
Fortunately there are not many things that are soluble in both oil and water.

If it's a water stain use an oil based undercoat, or a shellac based sealer.

If it's an oil, grease, or wax, stain use a water based undercoat.

Use two coats of undercoat to get a good seal.