I should note it happened to me a couple of times, or rather on two different grids it happened consistently After the first time, I saw the right-click thing, so I exited to the menu and hit play again, just to make sure I wasn't right-clicking a straight edge. But yeah, on the level I screenshot, rotating the curve in the lower right so that it connects directly from the raincloud to the fountain triggers the "level complete" animation, but restarting and putting it back into this configuration fails each time.
All of which I say only in case it helps you debug it. I can't make games at all, and I know game jams have added pressure, so color me impressed by what you've got!