Assuming you've assembled the fitting correctly on the line, did you *reuse* the same piece of plastic line? I typically reuse the line, but you may need to cut off the end part (that was cinched in the fitting previously) and use a new portion of the line for sealing. Otherwise, it's purely in how the fitting is assembled, use a new compression seal, and don't overtighten initially.. put it snug (remember it's a small fitting and a plastic line), start the car to build pressure, and gradually tighten until the leak stops.. If you overtighten you will likely cause a leak.
