In the final battle of Michael Bay's Transformers, Jazz, Optimus Prime's second in command, is torn in half by Megatron. After the battle is completed, Ratchet and Ironhide bring the fallen Autobots parts to Optimus, and inform him that they could not heal the damage done to Jazz.
During the battle, Sam had killed Megatron by forcing the Allspark Cube into Megatron's Spark, destroying both the Cube and Megatron. A single Shard of the Cube remained, which Optimus picked up from Megatron's body.
Earlier in the movie, we see the Decepticon hacker, Frenzy, is fully repaired from being little more than a head by simply being in the same room as the Allspark Cube. As seen in Revenge of the Fallen, the Shard is stolen by Decepticons, and used to revive Megatron. Why didn't Optimus use the power of the Allspark Shard to repair Jazz?