If you find you need more contrast beyond a #5 grade, use colder developer. I found I could push another 1/2 grade out of it for a 15 degree drop down to 55 degrees. I work in a basement, so my chemicals are that temp naturally in the winter/spring down there. Also, as Katie suggested, learn how to bleach to increase local contrast. At this point, don't worry about shadow detail as with thin negs, you can't be focusing on them. Just focus on the mids and highlights. FYI, developing at 21 should have given you more detail, not less so you might want to explore how you exposed your images as you likely didn't give them enough exposure.