Rear Case fan not working on motherboard

[Toby B]

Hi folks,
I have my board "GA-Z87X-UD3H"  installed into my case, and it seems to work with the other components.

However, the rear case fan doesn't work, even though it is connected into SYS_FAN1.   
The case fan has a Molex 4 pin. I plugged it into an adapter cable (that came with the power unit) into the board.

Have I plugged it into the wrong socket?  ... Are there known issues with fans with Molex plug headers?

I don't really want my CPU to overheat... so, any help you could offer me would be greatly appreciated.

Thanks for reading

[terraCrumb]

Some molex can have poor connections so try wiggling it with the power on.
A picture would be better explaining how you have it connected!