Could someone please advise me on flushing the water from the radiator.

All I can drain from the bottom bolt on the radiator is three litres of water and in the manual it says it holds 11 litres.

What am I doing wrong ?

Morgan 2009 Roadster Rolls Royce Lush Green 3.0 V6
Audi A4 Advant (some blue colour not botherd)