You have to understand that organics (compost, and whatever) degrade in beds. They turn into carbon dioxide and blow away. That's where carbon goes. Beds that are well amended with organics simply get smaller with time as the organics volatilize. That's just the way it works. Compost piles get smaller with time too! How to prevent it from happening? Amend with more organics every year. You don't need to uproot anything. Just pile on compost as mulch regularly.

If organics didn't volatilize, as leaves fall every year, we'd be sitting on miles and miles of fallen leaves.

Compost continues to degrade and break down, losing a lot of its volume in the process. If you add a high percentage of compost to raised bed, shrinkage of the material is to be expected.

You can get around this by using the compost only as a top dressing. Yes, it will still decline in volume, lowering the level during the course of the season, but is easily topped off at the start of each growing season. And you will still recognize the benefit of the compost using this method.

The alternative is to skip the compost and just use a mineral based soil with a well rounded fertilizer to supply the nutrient requirements.

Yes, if you want a bed that never changes in volume, skip the organics entirely. That's pretty much what one does in container gardening. But, as gardengal says, in that case, supply of nutrients is entirely from applied fertilizer.

Thanks for the help :-)