First of all you need to properly clean and oil the derailleur itself. As it is spring loaded, the shifting from top to the bottom ring envolves the spring to be pulled, thus making it harder the more you downshift. When you go back (upshift), the spring itself helps to do that and you feel the shifting much more easy. The easy way to do this: Take the deraiileur off the frame, but do not dismantle it. Get the new paintbrush about 1 inch wide. Get about 1 litre unleaded petrol (better is lighter fluid, but it comes more costly). Get a bowl. Get the lubricating oil (preferably with graphite). Put the derailleur into the bowl and pour the petrol over it, just to cover. Leave for a few hours. Then take a brush and clean all the dirt and grime off by using the petrol already in bowl. When done, dry the derailleur with kitchen paper towel and clean the brush. Get rid of dirty, used petrol. Now use the rest of clean petrol and clean derailleur again, then dry in the same way. When done, apply lubricating oil to all moving parts generously (don't forget jockey wheels). When done, get rid of excess oil using paper towels again until it is nice and clean. Fit back to the frame, re-cable and see how it works. If after all the problem still persist, then there is some fault in shifter itself. To sort this you will have to go on to the Shimano website and download instructions for your shifters. Hope it will help. Leave feedback if you are satisfied with my advice.