Waahnubis: Sorry, real life kept me busier than usual...

As already stated some scenes need a certain story progression before they can be unlocked, however you should not be able to spend points on them in the first place before the criteria are met. E.g. the scenes with the "Shark Mantic" can only be bought once you unlocked that particular mantic in Lily's lab.

In general Pandora scenes become available once the recruitment mission of the Waifu is complete - then the scenes show as locked in Pandora and can be bought with PP's (which you earn by increasing the Waifus devotion).

Once you unlocked the scene, however, it should be available to watch right afterwards. So if you buy a scene with points and then cannot view it, it can either be a bug or something I haven't encountered myself yet.

One additional point from my own playthrough experience:
Use different Waifus during the mission to evenly progress their devotion - else you may miss out one scene or two until you reach the end. I am now "missing" one still lokced Kili scene but have about 7 PPs left for Ela that I can't spend as I already unlocked all her scenes. I can spend these in the next release once more scenes become available, but for this release version that's it.