I've used several coatings if zinsser stain cover paint....no joy
Did you try an oil based paint, or an alcohol based paint such as Zinsser BIN?
The more you let us know, the more likely it is that one of us can help you.
Reading between the lines, you used a water based (Zinsser) product. If faced with that, I would use a shellac based paint such as Zinsser BiN or Blockaide by Smith And Rodgers (in Glasgow).
If you want to minimise the costs of materials, you could purchase a small bottle of "knotting solution". Like Zinsser BIN, it is shellac based but doesn't have any pigment in it (meaning that it is clear).
I don't want to come across as being combative, I am replying because I am trying to help you... however, when you ask the same question repeatedly, it means that "we" end up diluting our advice. That said, yeah, I understand you wanting to cover as many bases as possible.
If you do decide to try knotting solution (or BIN), either purchase a throw away brush, or buy some household ammonia to clean the brush. Ammonia is alkaline, it breaks down the paint and the brush will be as good as the day that you purchased it.
