Has anyone had the same issues... My DS is alm...

Has anyone had the same issues... My DS is almost 6 months and exclusivley BF except for the odd solid meal as required. I have been unable to express the last few months because no milk comes out with the pump. I can pump and pump and nothing happens but when my DS latches he feeds well. Does anyone have any suggestions? I go to work and would refer he have expressed breast milk if it is required rather than formula...