I think the question is, which flight planner did you use to create the ones that ARE working in PF3? It's difficult to say but in cases where some flight plans work and some don't, there is usually a difference in formatting.
Also some Win 10 users have found that their Windows regional settings can affect the loading of flight plans. I have no idea if this is an issue in your case but wanted to mention it. See
I've changed my win10 region to UK, with no results
yet maybe I have a further clue, to submit to you, in this witchcraft: trying to load every
flight plan I have, I am noticing that:
the flightplans over Australia are being loaded
over Middle East (Seychelles-Maldive-Dubai area) as well
over North America
- only one (?!) over Europe (so far)
so geography looks like to be a factor, isn't it?