Shape Builder tool not colouring

Hi everyone, 

I am in the middle on an Illusrator course at the moment and I am suddenly having problems iwth the shape builder tool.  Up until this point I have been able to colour my selected areas with no issues, however my latest assignment I have created an image, selected the areas to colour ( areas are greying out so I know that there are no gaps etc) yet when I press colour, the image is not doing so on my artboard.  The strange thing is if I save my document all colours are showing on the thumbnail of my assignment from teh home menu.  What is the issue and how can I resolve?