Well I know absolutely zero about tying and have no interest in self bondage but wondered if maybe the type of material your rope is made out of might be some of the problem??
It's poly-somethingorother, sold as "bondage rope". I have only ever had problems tying ankle to thigh - ankle-ties and above-knee-ties have been fine, even when tied as tight as possible.
(It's all just practice for when I find my next willing partner! :P)