feeding problem?

I am feeding my 5 mth old boy nearly exclusively. I feed him all day he has a bottle of formula last thing at night (on advise from health visitor) and then I feed him when he wakes in the night. Recently I have found that hardly any milk flows whilst I am feeding and my baby gets really distressed because he is not getting any milk. I've tried self expressing, pumping and all sorts. Could my milk be drying up? I don't understand why this is happening when I've followed all the professional advise. Please help!