Real World Flight Plans - beginner advice

I started getting into real-world flight plans.

I want to first try out the "vanilla Microsoft" approach, thus I want to make flight plans using the TrafficToolbox SDK.

Here is where I get tripped.

I created a arrivals/departures database from my local (major) airport's website, and have a real-world AFCAD for it as well.
(funny thing, after I did this, they changed the website and do not offer complete timetable anymore, I did it by extracting data with in-browser JavaScript).

Then I create a CustomRoutes-like file. Sure enough, it compiles. But 99 lines in, it fails to find parking spots. Now, here's the conundrum:

I get it can't find parking spots, the error is fine, but I find scheduled airplanes sitting on the ground 2 days prior to their departure (according to TrafficToolbox Explorer). If they're gonna sit at my airport, of course there won't be any parking spots. Why this? I have made the plan of all the flights for one airline with dual-leg circular rutes per the timetable, and tried various departure times, but they keep "sleeping" at my airport. Same if I reverse the route.

What am I forgetting here?

Thanks