Originally Posted by
waldowales
Check for a binding cable, either in the housing or under the bottom bracket.
In addition to this if the cable frayed inside the brifter, where it wraps tightly around the drum, the shifting will be
buggered. Loosen the cable at the FD and push the cable out of the brifter a few inches, any fraying will be
obvious, in messy cases it will be really hard to get the cable out of the brifter.