I presume this paper does not have developer incorporated, but take a little piece of it and immerse it in strong carbonate solution under room light just to see if my presumption is correct.

Old formulas for warm tone developers include a high ratio of hydroquinone and little sulfite. You can try adding a bunch of hydroquinone to the developer you have been using after diluting it more than usual. The bromide will help too. It may require longer printing exposure as well.