Quiver, try to acidify the paper with 1+30 diluted (in case of 30-35%) hydrochloric (muriatic) acid, it's more effective. Also, develop in drinking water and only then rinse in normal tap water. (Which - maybe - was gently acidified with a pinch of citric acid / few drops of hydrochloric acid...) Again, don't acidify paper if you aren't absolutely sure that it is alkaline buffered indeed, and still, opt to source non-buffered papers instead of acidification. (Student grade papers usually aren't buffered...)

Hope this helps,
Loris.