Thanks Ian for the reply. I didn't manage to get it working and I didn't want to make it too complex, so I dropped the idea of having one collection of collections and I simply created several regular collections.
The thing was, the sub collections were fields (means columns) inside main collection, so main collection had only one row and so even putting the main collection in a loop wasn't working to get to its sub collection members.
If anyone struggles with similar case, try checking Transpose Collection action in Collection Manipulation BO.
