The bath taps on my shower/bath mixer don't run?

Not sure about the make or model but water will only run from my shower head, not the taps. There's a silver barrel at the base of the hose which goes up and down and should switch between the taps and the shower, but water only flows from the shower no matter what position the silver barrel is in. The barrel also has two small holes in, one at the front and one at the back.