I normally ride all of the time on the center ring as it fits the roads here. A few days ago, I tried to put in on the outside ring (high gear) and it wouldn't go. I have Tiagra shifters and I just couldn't push it any harder. If I reached down and pulled the cable with my hand, I could get it to go. My first thought was that the cable was too short or loose. Back home, I was able to push the DR over the high ring and could see that the cable was a little slack. I tried several times to pull the cable tighter but just couldn't get it to where I could shift to high with the lever. I put the chain on the small ring, pulled the DR to the high ring and wedged it there and loosened the cable retainer and pulled the slack out as tight as I could and tightened the retainer. The adjuster at the shifter is screwed all the way in and I tried to tighten the cable with that but no luck. Not sure where the problem is, but I think the cable is worn inside as there are several needle-like strands sticking out from the end. Could that be the problem? Thanks

See if this will help . Repair Help Articles - Park Tool

put in a new cable if you dont like the frayed end of the old one.

Those needle-like strands, could they be coming from your cable housing? If that's the case, they're your problem. You need to replace the housing or at least cut the end nice and square and use a snugly fitting cable housing end on it. If you don't ddo that, your cable housing will continue to gradually shorten itself and you'll never be able to shift into your biggest chain ring.

Yes, the fine wires are belonging to the housing that the cable runs through. The housing has some light bending as well as if it was bent and someone tried to straighten it but wasn't successful in getting it back to original. Thanks

then it best to replace the housing .