have you tried switching which foot goes in front? i could not get up until I switched and went with my left foot forward. I also have my back foot (right) on the ski just not in the binder, then slide it forward. does your foot fit the rear binder when not in the water? or is it you just have trouble geting the back foot in? try just getting your back foot on the ski, see if you can stay up, then worry about getting the foot locked in.
oh, is this a individual slalom ski or part of a combo set? some of the individual slaloms are designed for fancy stuff and harder to get up on, until you get the hang of it.
on the combo sets you'll notice the slalom ski usually has a larger fin on the bottom for stability. if you're a big guy, you may want to see if this fin is changeable, and see if you can find a larger one to help until you get used to it.
just my 2 cents....