No cold water in the shower?

I have just had a shower installed and had a number of issues with the builders/plumbers so won't be getting them back.

We have hot water coming through the shower but No cold. Opend the valve in the loft and can here water coming in but then it doesn't seem to reach the pump downstairs that then pumps it back up.

When we turn the hot water on the pump kicks in and pumps it back up but it seems as if the cold water isn't reaching the pump as it doesn't kick in.

Could this be an airlock and how do I remove it and get it sorted?