Where's the Flavour Gone?

I have been making all of my beers using Extract Kits.

When making them, I occassionally add extra stuff to try and improve the flavour. To a stout, I have added a bottle of Ginger Beer. To a Bitter, I boiled up various spices in water and added this instead of 'plain' water. To another Stout, I bewed a few pots of coffee and added this.

In all instances, the beer has tasted really nice however, after several weeks, the added flavour seems to deteriorate. So far, only the coffee stout seems to have kept the flavour that I wanted. Why have all the others gone?

Thanks,