Who is developing your film and who is producing the prints?
In general, low contrast in the negs will result in a grey print unless account is taken of this and the print contrast increased. Ideally, you want to produce negatives that print at grade 2 with the contrast that you want. This is essential if you are getting your prints made by a lab, less so if you are printing yourself as you can then change the paper grade to suit the negative (within reason). Two things determine the ability to print at grade 2: negative exposure and development time.
The Les Mclean book Suzanne mentions, although not big on formal portraits (there are a few) will show you how to spot under/over exposure and under/over development and is a great general introduction to serious B&W photography.
The general rule with B&W is, when in doubt, to overexpose rather than underexpose (you tend to lose the shadows as you found out) so try rating the 100 ISO film at 50 and take it from there. If you are still getting low contrast, try a 15% increase in development time. Better still, forget everything I wrote above and just get Les' book...
Cheers, Bob.