If this fails there are more drastic techniques.
Once emulsion has really 'gone off' the hot water and soap may not be enough. If possible keep it wet and wash it out is certainly best.
Once set, "nitromors" will soften it back to being washable, but its not nice to some darker fabric dyes - a careful test on a bit in the corner with a cotton bud is called for.
If the dye is not bleached then dobbing nitromors over the affected areas, waiting until soft and then washing out should work.
In extremis, if its just a few odd spots, consider staining the areas of emulsion to match the existing curtain colour!!
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