Hm.. it used to do that. Approach one of the basins, it should give you the option. Also check google for which quests award a shard and make sure you have done those. Each reduces life of Sinmur significantly. If I remember correctly, having and using all shards makes him even susceptible to CC...
You get the shards from doing the quests leading up to Sinmur. If you have them, you should see an indicator to use them.
However, I have beaten this boss with no Shards at all. The trick is to let the NPCs take all the aggro. They won't die, the worst that happens is they go to a knee for a period of time. So, while the NPCs are keeping the boss occupied, use ranged attacks and stay away from Sinmur. It takes a long time doing it this way, but I prefer it actually, the shards makes the fight go too fast. Note there will be times when all the NPCs are taking a knee, then you have to run around like Benny Hill until they recover.
Enjoy the game, life is what you really want to be worried about.
PakKat "Everything was going well, until I died"
Gary Gravestink "I am glad you died, I needed the help"