There is a technique called "dye dodging" that may be helpful in these situations.
Fix out some unexposed and undeveloped film, wash, and let it dry. Bind a sheet of this clear film to the non-emulsion side of the negative. The aluminized tape used to bind slides works fine for this purpose.
Working over a lightbox, apply a dye to the clear sheet of film in the shadow areas that are too thin. Try to make the application as uniform as possible. A Q-tip works fine for larger areas, while a fine brush might be needed to work details. Let the dye dry, and then make a test print. Apply additional dye as required.
I use Dr. Martin's Transparent Watercolor dyes for dye dodging. Magenta tends to work well, especially if you print on variable contrast paper. The magenta dye acts to hold back light, while also increasing local contrast allowing detail in the shadow to appear in the print. Yellow dye will also work - it has the affect of reducing local contrast.