I've tried for a couple of days to recall the specific case that inspired this question so I could ask it in a less vague way, but I wasn't able to, so here goes:
What circumstances could lead to a peer proving succession to a subsidiary title, but not their primary title? I'm almost certain the example that I'm thinking of involved a peer inheriting all of his titles from his father, all with the same remainder, but per the Roll of the Peerage and their Wiki page, they had only so far proven their claim to one of the lesser titles. If I'm imagining this, or if the case I might be thinking of involved a title with a special remainder, I'd be happy to stand corrected, but it's been bugging me for a couple weeks now and I haven't been able to find an answer