low pressure light comes on and the shower gos cold ?

I have just had a triton rapide 4 shower fitted which i have had in the draw for about 2 years so the warranty will have run out, It was fitted about 8 weeks a go when i turn the shower on it just it just go's cold and the low pressure light comes on it is my second triton shower so i might be able to take a spare part off the old one ?