Hard Piping and Compression fittings

Im doing a watercooled set up with hard pipping. The hard pipe is cut a little bit longer than the opening of the fitting. Reason why is the pipe need to stick into the fitting. Im having a hard time getting pipe into the fitting. Any pointers? Am I doing somthing wrong?

Put the pipe in before you mount the hardware maybe?

Are you certain the pipe and fittings are the same diameter? One is not metric and the other SAE? I use Monsoon fittings and pipes. The fittings fit snugly but not overly tight.