Pips, just saw this thread and want to say sorry you're having trouble with the straps! I've never had them not hold any knot, even when new, but I'll agree that after you've been hanging with them a couple of times they do hold the spike hitch better than the very first run. I've found that when you are rigging the straps up and tying your knots if you really crank down on them before you try to hang the hammock they will hold the knot. They just need some persuasion the first couple of times.
Sounds like these guys have also given you some good suggestions. If you keep having problems with them I will send you a set in a different webbing, just let me know.