Seriously, I've had Fruit Flies trying to go up my nose and in my eyes be less frustrating and annoying than this was!

Full details provided here on the Shared page.

The TL;DR cliff notes:
- Problem is while trying to get the TOP of the Pink to connect at the BOTTOM of the Blue part. (Or, the BOT. of Pink to TOP of Orange, but really its the same issue)
- Mating surfaces are the same sizes, parts are at the same scale, and identical rotation
- It's not impossible, but it's damn hard, requiring certain specific camera viewpoints and distances.

NOTE: Just in case, I do not need these connected now, as I did finally manage it. I'm just reporting this for the actual issue. Masochists:feel free to torture yourself by trying to accomplish this!

However, I will say, IF you do manage to, and do so with minimal effort, that would indeed be valuable data for the devs, to see if it's related to a particular platform. In that event, leave a comment with your game version and platform (iOS, Android, Windows, MacOS, Linux[?]).
My system: Windows 10, SimplePlanes v1.10.106.0, using a Mouse, playing in Fullscreen at 1920x1080 and maximized settings in DX12 mode (forced).

Formalities

Player.log, ModLoadLog.txt, settings files, and even ModSettings folder with its contents just in case.
https://cdn.discordapp.com/attachments/892893423378497558/895437245564207164/Logs-n-Stuff.zip
(Though there's no errors to go along with this. Just plenty of the vanilla clutter-error when you set a part's Input to "Disabled" through the Part Settings menu.)