Cavity wall window

Last year I had a window installed on the gable end side of my house. The wall was knocked through to the cavity wall then the outside brick was removed a lintel put in and the window was fitted. Not long afterwards i noticed whenever it rained water would be deposited all over the inside window cill and finally i noticed the plasterboard on the inside of the window was wet through and i could push it through i have since had the window replaced and on the odd occasion there has been water coming down the inside of the window. Anyone have any ideas what could be causing this problem?