Is my ragu missing an ingredient?
I do agree with the comments about using the proper temperature and simmering. It can be difficult to achieve that perfect simmer, especially with a lidded pan, but quite worth the effort. 

Please see this from  CookingLight  as it gives very good information about boiling and simmering and how to get to where you want to be.

Another thing would be to check on it more frequently (after achieving the simmer). 500 ml of liquid is not a huge amount, especially with a 2 - 3 hour cooking time. (You will surely have some evaporation, even with a lidded pan, and the amount can vary from one time to another.) If you see that the liquid is getting too low you can add more.