I've got the Avent manual pump and can usually get about 3oz in one go over 15 - 20 mins but only in the morning. Today it's taken me several attempts over the past 2.5 hours to get 3oz. If I express in the afternoon / evening I can get 1.5 oz max. Am I doing something wrong or do I need an electric pump? Would ideally like 2 bottles in the fridge each day and some to freeze.