Don't worry Gomer, I understand what you mean. I can attempt to answer why you don't feel fatigue, and how to correct that.
I think you don't feel fatigue because you stretch more of the tunica when hanging SO opposed to the ligs when you stretch BTC or SD. This is not a bad thing, but it may take different amounts of stress to deform the different tissues. Also, if you are using a hook or just draping a rope over a PVC pipe, 18 pounds of weight might not be even close to 18 pounds of pressure after you figure in friction. Increase the weight to whatever is comfortable and see if you can feel any fatigue from that.
Also, did you see gains while hanging at the lower angles (BTC & SD)? If you were gaining, did you see a halt in gains, and is that why you switched to hanging SO? Or was the fatigue just too great, and you decided to hang SO for a day or two?