I had that problem last year. This year, my make-shift solution was to cut a hole in a small scrap of cardboard just smaller than the rebar and force it over the rebar before sliding the conduit in place. Not sure if I described it well. It is working okay, but I imagine it will break down and the conduit will be able to slide down through as the season goes on.

I used a 1" thick x 4" x 4" piece of treated wood. Drilled a small hole in the center just big enough to slip over the rebar. Then slipped the conduit over the rebar. This keeps the conduit from sinking into the ground. The cardboard might work unless you leave your conduit up all year. If you do, the cardboard will be mush by the second year (if not before). My trellis has been up for 3 years and the wood is still holding up.

^I do take my trellises down during fall garden clean up. The cardboard was a bit of an experiment using materials I had on-hand. Great to know that wood would work the same way for a more durable result!