I think I worked out why I was having such a problem with my lift off and not getting the height I wanted/needed. I tended to try and lift straight up, my legs seemed so heavy and almost impossible to lift high enough to jump back through (picture on the left is how I used to approach it. Notice my arms marking a straight line from my hips to my shoulders and even my head with my legs stuck out front).
However in Utpluith (Tolasana in Swenson) I seemed to be getting so much more height. Same thing when I worked on my Lolasana. I noticed that in these two postures my shoulders were further forward counter balancing in lolasana and with my weight more central in Tolasana.
I checked the literature, Swenson has the the straight up lift in his book and I think that's where I got my original approach to the lift from. However if you look at John Scott's book,http://www.ashtanganeworleans.com/Old%20Website/StudentSection/Jump-Back-JS.htm he stresses leaning forward to lift up. This seems to make much more sense, especially for the beginner as the weight of your legs is further back closer to your arms where they are much easier to lift.
This is how I'm doing it at the moment. I'm finding it much easier to lift up allowing me to jump back through but perhaps I'm still not getting my shoulders far enough forward which might explain why I still tend to brush my toe along the mat.