Bamthwok, it indeed doesn't work if you backed and layed with the same amount, just like the opposite: if you back and lay to a profit on a single selection, you wouldn't be able to grene up, you would need to use the separate button "Distribute Profit".
We'll think on what can be done. Right now you can do it manually by backing on the selection with the loss, with the following parameters:
price: the current back price
amount: -PL/back price
So if the price is 4.0, and the PL is -30, you would back 7.5 to even the loss on all selections.