help sick cacti

Hi Im wondering if any one can help, I have a four foot tall Peruvian torch type cacti that I repotted last may and it has been dying at the tip ever since. I scored the root system at the time and put it pretty deep in the pot. Is it possible that I put it down too close to the drain stones? Any feedback will be greatly appreciated!!!!!

 

Can you describe the dying tips? Dried-up or slimy? Brown, black or something else?

 

IT is drying out at the tip about a half inch at a time it is not wet looking or rotten. any ideas?

 

No, but I have seen a similar phenomenon on one of my cacti, it slowly dried up from the base. Finally, I cut off the top upper part. The tissue looked healthy and the cutting rooted as it should, and I still don't understand what happened to it