I experienced something similar my first time using a supercharger in DE. Turns out there were already 4 MS there, and I was the 5th and had to wait a short about of time for a spot (only 10 minutes or so). When I plugged in (with 3 others still charging) it was pretty slow charging at first, but pretty quickly ramped up (as the other MS had been there at least 20 minutes+). Got to chat with all but one of the other owners while I was there which was fun.
Interestingly enough I've been back to the DE SC a few times since and have always been the only MS.