If you put a little pipe dope, grease, or some other kind of lube on the threads of the compression fitting, it will make it easier to tighten down smoothly. This is not a sealant, but merely a lube for the threads. Also, when you tighten down the fitting, pull the tube out just a little bit. As the compression nut is tightening down, it will try to pull the tube in at the same time. By pulling out the tube, it gives some room for the tube to move in to the fitting. Tighten the fitting just tight enough for the fitting to hold and then about a half turn more. If it leaks, tighten a little more until it stops.