Edward the Black Prince and Lionel of Clarence both died during their father's lifetime. It was understood that a male grandchild would inherit his father's place in the succession, but not a female. As such, Richard moved up one slot and became the royal heir, while Philippa of Clarence was ignored, little thought to her marriage, her issue ranked second to the princes of the blood, etc.
Well, I don't know that much about English succession, so I ordinarily would agree, but even if that was a reason for Philippa's claims being ignored (and I've never seen it even theorized that it did - as far as I know, the primary reason for Henry IV's claim of the throne was that he was the heir under Salic Law while Edmund Mortimer was a child and descended through a female) it makes very little sense. It does make sense that Philippa's issue was ranked second to the princes of the blood because ducal titles don't pass through females and thus Roger Mortimer was only an Earl of March, whereas Henry Bolingbroke was the heir to a Duke. I have heard that Edward III instated Salic Law and Richard II later abolished it, but I doubt the effects of that would have been different had Lionel of Clarence still been alive in 1377. I'm trying to remember a later English royal whose second son left only daughters whereas his third son did leave heirs. But for example when the present Queen Elizabeth married, there had to be a special law making her children princes and princesses, whereas her Gloucester cousins and their issue had that automatically. So if that still happened even in a time where male-preference primogeniture was widely accepted, I don't think Philippa's children having a lower rank than those of the Duke of Lancaster necessarily proves Philippa's removal from succession due to being a female grandchild. So if you don't mind, would you care to show me something to verify your argument? I would be interested in seeing that, because it would mean Henry had a (flimsy) claim that didn't just consist of him getting drunk one day, proclaiming he was King of England, and the Lancastrian nobles around him thinking "Sure, let's go with that."