plugs can get worn from being outside, lack of lubrication, and just age. sometimes they have been rekeyed several times, with reamed out chambers, or have had the tops of the plugs filed when someone tried to use oversized top pins. if there is excess space between the lug and the cylinder, it will be hard to get pins to set. try varying the way you apply apply tension, top, bottom, etc. once in a while that will do the trick. on old, worn cylinders you might need to use a rigid tensor to get better control. i'd take the cylinder apart examine it to see if the plug has been worked over. if it's all you have at the moment to practice on, give them a good cleaning with a solvent to get rid of all the built up graphite that might be preventing you from feeling when a pin sets. most locks will pick easier in one direction or the other, depending upon the binding sequence. it's good practice for 'real world' situations to try to pick it in either direction, an well a more of a challenge.