I've only read up to a pointt, but I wanted to add m own experience as well.
My dd lived in the uk for 5 years (where she was born) and went to nursery and reception class there. We have always spoken Italian at home, and have just come back to Ital where she now attends an english school.
We find that her 2 languages are for the time being almost equivalent.
From the (limited) experience and reading OI have had on the subject, I also think that it's a lot more difficult when its the father who speaks the 'foreign' language, as he tends to spend less time with the children (usually).
I have also heard of alot of children not wanting to fit in, or being different, and this might kick in when they're older, but I think my dd, growing up in London, found it quite normal as at least half her classmates had a second language at home (even though they were perfectly fluent in English) and now she's in an international school, so children who can only speak english are actually a minority.
This is he only explanation I can find, she has always been proud of being able to speak 2 languages (somrtimes braggibg about it, I'm afraid), whether this happened by accident, or because we have been able to convey this to her, I don't know, but I do think it might make a difference in some cases.