Andy_Langton - 1:06 pm on Dec 21, 2012 (gmt 0)
I've certainly observed this problem in many different situations over the years. One of the factors involved is Google's increasing reliance on 'authority' and safer-bet results. And for most sites, the most authoritative page is the homepage.
'Cannibialised' rankings can be a tricky one to resolve. Two obvious steps are to improve the authority of the landing pages themselves, and to ensure the homepage is not mistakenly targeting the same keywords you want to rank for with a more specific page.
The question that's difficult to answer, of course, is whether your landing pages would actually rank back where they were, or whether they are no longer considered strong enough candidates.