Bath mixer taps with shower - no flow from spout?

Hi guys,
I have installed a brand new Century bath mixer tap with shower hose in a brand new bathroom installation running off a combi boiler (1.5 bar pressure).
Initially all was well but after a day of service the spout does not allow water from the hot and cold to flow (only dribbles), BUT when switching to shower only, hot and cold does flow very keenly through the shower hose. I've re checked all pipework, isolation valves etc. and think the problem appears to lie in tap, could there be an air blockage in the tap valve to the spout? if so how do i rectify? Basin and shower taps all work fine.
Thanks,
Jeff

either the mechanism to switch from bath to shower is faulty or there is a blockage in the spout, can't be to hard to check the spout by taking out shower connection and blowing through it, possibly polystyrene packaging got into the taps and have blocked the spout at the end

Panic over - took the spout apart and found a piece of soggy cardboard in the spout end causing the block. It's easy when you know how!
Thanks for the reply.
Jeff