This issues are there since last year in summer. It is not something new. Mostly on weekend, summer and holidays there are mostly no seats left for pass holder. There is only an amount of seats for pass holder. We in the community advice to book early.

Best way to book the reservations and to check if there are currently seats left via b-europe:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

As soon as you decide to take Eurostar, you should try to book it.

You can tweak availability a bit by booking shorter stretches. E.g. If you want to go to or from Amsterdam but it appears to be fully booked, try if there's still availability to/from Brussels. Easy and different connections to/from Amsterdam are possible via Brussels.

Same counts for the one to/from Paris. You can always try to take a Eurostar to/from Lille, which as plenty of connections to/From Paris with ordinary TGV's.