Can't dock in Artemis Bridge Simulator

I sponsored my first Artemis session at home last weekend. Everyone on the bridge other than me was a newbie. We ran into a recurring problem of not being able to dock.

When I've seen this problem before, I attributed it to being at the wrong vertical height. So I just moved up/down until I found the sweet spot.

During our recent session, this did not appear to be the case. No matter our vertical level, the DS wouldn't tractor us properly.

I made sure out helm programmed 0 velocity, was within a range of 500 m (sometimes we were as close as 20 m), and we tried many elevations. The tractor started but stopped almost immediately. The only way were able to restore our energy levels was to spam the dock request hundreds of times.

So, what could be causing our docking problems? How can we avoid them in the future?