Does the chain skip over the 13 when going from the 12 to the 14, or from the 14 to the 12?
If the first case then too much tension, in the second case too little tension or dirty cable housing.
Try lubricating the cable through the short housing just before the derailleur. If you loosen the cable you can slide the short housing up the cable exposing the cable for lubrication.
Are you able to shift to all other cogs?
Is the cable routed correctly at the derailleur pinch bolt?
Al