No water coming our of the showerhead

[thegancio]

I recently had my bathroom refitted and we installed a shower in place of a bathtub, the showerhead is a small rainshower fitted in the ceiling. Lately every now and then I go to turn the shower on and no hot water comes out, I turn the valve to cold water and water comes out. At the same time there is hot water coming out of the tap in the same bathroom and kitchen. The annoying thing is that we called the plumber that installed the shower twice and every time by the time he showed up the shower worked.....any ideas?
In the same shower we also have a small shower head on the side (To use to clean the shower), this is much lower than the ceiling shower but still no hot water coming out.

 

[Technotim]

More information required.

What is your hot and cold water, mains or gravity or gravity pumped ect.
What shower valve is it make and model ect.

I can guess that the pipes come down from the loft and its air but it would only be a guess without more info.