I cant relock my z3 bootloder

hi
i have unlocked my bootloder on my sony z3 dual and now i want to relock it so i can update the phone , but i cant.
i used a simple methode using the flashtool but it didn't work for me even though i was using the lastest flashtool.
so is there a soultion fro my problem or another methode that i can try ?