I had the same issue for last summer . For those not familiar with the CDI system , the halyard does ride in the groove .
There is no chance of a halyard wrap ,so common to other furling systems .
I beleive the actual problem is there is a part missing . There is a small black plastic triangular piece that rides in the groove .
My inquiries to CDI lead me to believe there was not an available replacement . I made several out of Cpvc,pvc, stainless steel and uhmw .
The easiest one to move along the slot was uhmw . The halyard attaches to one end of the triangular piece and keeps the halyard inside the groove .
The other end of the black plastic groove keeps the number 6 sail tape inside the groove .
I believe what you are experiancing is the halyard is rubbing on the groove as well as the sail tape is binding in the slot .
Let me know if you aren't able to locate a replacement block.
I just looked at the drawing for the CDI parts from an earlier post. The black plastic triangular piece I spoke of is not listed .
On my Oday 25 I use a CDI ff4 . Perhaps the piece isn't used on the smaller units . I would be tempted to use a smaller diameter higher strength halyard .
Reviewing the CDI reefer posted above , the part I'm refering to is the sail slide . This helps keep the halyard and the tape inside the groove