the current implementation of the arrival / departure detection only detects departures as soon as the aircraft takes off again. So if it arrives, stays on ground for 1h and then takes off again, the arrival will show up only after 1h. Since most aircraft stay on ground longer than 15 minutes you're getting the empty result set. I'd like to explain it to you in more detail, but that's all I know for now. So no, you're not doing anything wrong, that's just the way arrival detection currently works.
our heuristics also depend on the coverage of receivers next to airports, which is something we can barely influence. We haven't found a way to make it much better without flight plans. So, for the time being I wouldn't bet on great improvement.