Is the EU's attempt to reduce CO2 emissions failing?

The European Union Emission Trading Scheme was introduced in 2005 to combat climate change and create an incentive for power plants and large companies to reduce their CO2-emissions.

The idea was to introduce a price on each tonnes of CO2 emitted.

But has it worked?

The graph here shows how the price of emitting CO2 has dropped steadily over recent years, meaning that the economic incentive to cut emissions is also dropping.

If the system seems to be failing then what should the EU do to create more of an incentive for companies to lower their CO2-emissions?