What is causing this stain

[Ooogetyooo]

Newly plastered internal wall , allowed to dry out fully and then painted about 3 coats of white about 8 weeks later. A few days after this a horizontal yellowing stain started showing through. Have then sprayed on a stain blocker and given it another coat of paint but it keeps showing through. Stain doesn't feel damp so what could it be?

Something leaking within the wall or penetrating damp? The moisture could be pushing salts to the surface. You have to let something dry out thoroughly before you stain stop paint or it will show through.

yellow stain is usually water, still present, but can be tar leaching from an old chimney (which will in time go brown). Not condensation, but a leak or rainwater. Tape a piece of clear plastic tightly over the stain. If mist or condensation forms on the wall side, it's a wet wall. It might not feel damp if it is evaporating at the same rate it forms.

What is on the other side of that wall? A bathroom?

Why did you need to have the wall plastered? Has it been treated by a "damp-proofing" company?

Is it on the ground floor, and is it on a concrete floor?