I had one last year doing the same thing. After second (zero time) PLR we started seriously examining the prop, and found the seating area in the barrel nut to be worn. We changed the barrel nut and problem immediately went away. Basically, the barrel nut had been reworked too many times and the seals couldn\'t hold pressure any more.
About 6 weeks ago another one refused to lock. On investigation we found the PLR index rings to be badly placed. The real reference mark was almost invisible, and another had been made one tooth away ......
As a rule, I always use 98% rpm as minimum torque reference. First attempt from 850TIT, second attempt from 925TIT, last attempt from take-off/1077TIT. I have had successfull pitchlocks as low as 103.4% rpm on first attempt.