So, I realized that there was a problem though I'd call it a bug.
I was not able to call that particular field but I was successful with all the others belonging to that group.
I don't know why.
I realized this only after rebuilding all the site and application from scratch and renaming the fields exactly like in the first site... the only difference was that this time, to avoid the problem, I just put everything into the same content type, no more group.
But... nothing, I wasn't able to call it!!!!
Maybe the name was too long? or what???
Not that it matters right now since I renamed it and it's working but I think it could be useful for others
the name was something like this
xxxxxx_xxxxxxx_xxxxxxx
Too long maybe?
No idea.