Bissell Quickwash shampooer 1950

My old shampooer (Hoover Aquajet) packed up on me so I decided the purchase of a new one was in order.
I live in Milton Keynes so we have a number of stores to choose from, and I went to Curry's and purchased the Bissell 1950 model for £79.99.
When I got it home, the instructions were very clear, but I fear I am doing something very wrong as the water doesn't seem to want to come out when I squeeze the trigger, unless I hold down the shampoo container, which is not very comfortable and I think I might need to take it back to Curry's to exchange or for them to try it to see if I am doing something wrong.