Is it possible to provide screenshots or a video from prior to and during the descent, please? That'd help us figure out what's happening.
Just a clarifying note on some differences between the 737 and Challenger:
1. VNAV isn't a descent mode unto itself, rather it's a modifier on the other modes, e.g. FLC, V/S, PATH, and PTCH. Enabling VNAV will allow the FMS to control altitude preselection to meet flight plan restrictions, and also speeds if separately enabled.
2. The descent path calculated by the FMS is not an idle power descent as is often used in airliners. The Challenger will fly a geometric path, based on the default descent angle set, a per-leg override, or what's required to meet a restriction.