Front derailleur shifting issue's

Have you got the right chain? Some years ago, I ran a Wipperman/Connex on my Campag kit and found it would not climb onto the big chain ring as fast as I expected. To get over this I put more tension on and set the derailleur cage to move a tick further to the outside with the net result that the chain was forced to climb but occasionally dropped off on the right. I went back to a Campag chain and reset things and everything worked fine.

The Connex chain was nowhere near worn out so I tried it on my mtb shimano kit and it would not climb onto the big chain ring there as well as the shimano chain either. Binned it!

As Kil0ran suggests it now sounds like you need to set the cable tension using the cable clamp bolt. Shift onto the small ring, loosen the cable clamp bolt and using a pair of pliers pull the cable tight then do the clamp back up whilst keeping the cable tight. 

Most likely cable tension if nothing has been knocked out of position. Sometime chain maintenance can also spoil smooth shifting, is the chain well maintained?

If you pull the cable manually (assuming you've got external routing) will it complete the shift? If so then it's definitely tension.

Wind out the barrel adjuster half a turn at a time with the chain on the 3rd-to-biggest sprocket and try the shift again. Repeat until it's clean. If you run out of adjustment, wind the adjuster all the way back in, then half a turn out, then disconnect the cable and reattach. Pay attention to which side of the pinch bolt the cable enters as that's critical to a clean shift.

Barrel adjuster will either be on the frame near the head tube or near where the cable exits the bar tape. If it's on the frame double-check you've got the correct one as some bikes get set up with the cables crossed (i.e. front mech cable runs to the right hand side of the frame)

If it is tension it won't need much adjustment if it completes the shift on the smaller cogs. Half a turn and repeat until it's clean.

Unless someone has fiddled with the limit screws the only other thing it might be is that the derailleur has taken a knock and got slightly out of alignment. Fixing that is a bit more involved - read the Shimano dealer manual (assuming it's a Shimano FD)

If the problem has only just appeared and the shifting was ok originally then I won’t be mechanical adjustment of the Low and High limit screws. Before you start playing with these, if the shifting was ok at one time then try cable tension, after all, how likely is it that the limit stops have moved themselves?

Its not cable stretch insofar as adding more tension isnt the way to deal with it. Put it on the small ring and biggest sprocket at back, detach the cable, check that the derailleur doesnt rub at the inside. If it does, adjust the L limits. Wind the barrel adjuster all the way back in, wherever it is. Pull the slack out of the cable, reattach it, then retension the cable with the barrel adjuster again until you get a positive shift. Now check the H limits, so that you dont have any chain rub when in the big ring and small sprocket.

Somewhere you will have a cable adjuster, wind it out a few turns and see if that sorts it out. Cables don’t stretch but furrules bed down so you shouldn’t have to do this too often