HE: which know is the tight one, and which the loose one that lets the slippage happen? Are you getting more or less slippage with the ascender?
What you describe with the ascender knot is why many of us went to using them rather than prusik's for climbing. They slip easier when you want them to slip, and hold well when weight is put on them. You just need to know that it is easier to make them move accidently when not under a lot of weight. Of course, this isn't the life and death issue with the way you are using them that it is in a verticle caving climbing rig
. For this application, it seems that it would work well where folks have been using the prusik.