Damp stains bleeding through stain block.

[MacLaverty]

have 2 old chimneys and an attic bedroom which has half of these chimneys plastered with lath and plaster. Recently, these chimneys have got damp through the storms and stains showing through the paint. I have now used both Zinsser cover stain and Zinsser Bullseye 123 yet the yellowing is still bleeding through both within 24-48hrs. Why would this be happening?

Do fresh damp stains bypass block stain or should block stain stop all staining whether old or new? Checked the back of the lath in the loft and it's bone dry and non-moldy so no idea what's happening.