Looking at the questions asked by Huw and by Ian - I wondered if it might be worth using an expression like "in the process of …" to distinguish between the two cases.
e.g. "Los libros estaban siendo colocados en las estanterías" >
> means (more or less?): "the books were in the process of being placed on the shelves"... It really does need some kind of special treatment, because there is not always a directly translatable equivalent in English (although here, we could actually say: "the books were being placed on the shelves"). The above sentence could usefully be compared with
Los libros fueron colocados en las estanterías
and
Los libros eran colocados en las estanterías
and
Los libros han sido colocados en las estanterías.
and [maybe !]
Los libros habían sido colocados en las estanterías.
Of course, each of those^ alternatives carries a different meaning.
[… What about: "Los libros estuvieron siendo colocados en las estanterías"? !]
-----------------
In your example: "The strawberries had been in the process of being collected" is probably a better illustration for my suggestion.
