There are three key steps to making leak proof copper pipe joints.
Clean off both the inside of the elbow and the outside of the pipe with steel wool or a special cleaner tool.
Apply a soldering flux to both cleaned surfaces. The cleaning gets rid of contaminates and oxidation on the metal that could prevent good solder adhesion and the flux prevents oxidation while heating the pipe.
Hold the torch on one side and the solder on the other side. You want the metal, not the flame, to melt the solder. That way the solder will actually be sucked up into the joint. Don't get it too hot or the solder will flow back out. So use a moderate flame and be a bit patient, so that when the metal reaches just the right temperature but not too hot, the solder will melt and be sucked up. Then simply remove the flame and slide the solder all around the joint. You will see it melting and going into the joint, uphill, horizontal or down hill, doesn't matter.
If you still have problems it is probably because of the change from lead to lead-free solder. For details on solving that problem, click here.