I've got an old bike I'm going to sell on ebay. However I can't get the gears to work properly. At the chainwheel, it goes from the high chainring (3) to the low chainring (1) but won't go from the middle chainring (2) to the the low chainring (1). At the rear wheel it will go through all the different sprockets no problem on all three chainrings. So I therefore think the low and high adjustor screws on the rear mech are okay, and that I also don't need to do anything further with the adjustor barrel (I did play with the adjustor barrel initially but then the chain jumped off the top chainring etc). Could it be that the high and low adjustor screws on the front mech needs adjusting? I do recall fiddling with everything at one point even the front mech. Any alternative suggestions if this isn't the case?