I've learned this the very hard way by spending quite a lot of money on what I thought was a good board with 5 slots - 5 extender cables and GPUs, only three of the GPUs are being recognised. I'd check first - certainly before butchering your case (if it's a nice case)...
And remember that GPU cards always seem to run cooler *outside* of a case than inside. Even without box fans etc. my open-frame rig is cooler than the case boxes.
Can you share which mother board you used?
Coz, i am going to buy gigabyte 890FXA-UD7 AMD mother board with 6 PCIe slots to connect 6 cards with riser cables.