This a subject I did studied when writting my first book.
The story can be summarised like this. Most of the songs that went into the two albums basically had collaborations by other band members, some of them changed quite a lot, for example, Floating (A.K.A Purgatory) and some not so much (Wrathchild). At the time, most of those members who did not remained in the band, basically had no interest in protecting their contributions, they just did it and didn't think much about it because, after all, to them, Maiden was just another pub band.... Enter Rod Smallwood (around mid 79) and things are growing so he goes and basically sets the rule of 'you're not in the band, you're not credited'. Now, some of the past members (as was Blaze) were compensated for their contributions (Bob Sawyer, Sanctuary), while the others just didn't care. Of course, there's the exception of Barry Purkins A.K.A. who did contribute to what we know as 'The Ides Of March'... But what happens? Maiden has a huge record label behind them while Barry has almost nothing so what was he going to do? Litigate? No chance... So he basically got sweet F*** for his contribution... Somethings similar happened to Dennis Wilcock with the exception that his lawyer was a total c**t who thought he would be smart and yes, did get a settement but nowhere near what he was claiming...
Anyway, what I'm trying to say is that most of the time, things have been settled in an amicable way and there are no hard feelings... But there are some exceptions like Ron Matthews who sometimes is bitter about how his tenure with Maiden ended....