GDR2020

Hi. Yesterday I was doing a flight from KMSP to KORD and when I was asked to descend, I entered a nice low altitude (3000 feet) into the FMS just to ensure I would capture the glideslope, and pulled the knob and the it went into P. DES mode with the throttles at idle (RETARD) as it should, and descended bang on the V/DEV profile (like +/- 30 feet). I was assigned a STAR by ATC and it was around here that I ran into trouble. it went down to the first constraint on the way into Chicago and refused to go any lower in that mode. I ended up having to use LVL/CH mode to get down. It's a really short flight and easily able to be replicated if you would like a video. I guess thinking about it, I never activated the approach phase. Would that have something to do with it?