No expert, but if you add soil doesn't that mean you'll need to remove some to make room ? Seems a better plan to me to add whatever nutrients are needed to the soil that you already have.
Well rotted farmyard manure is always a good tip.
If plants are still looking poor, you could add some slow release, granular fertiliser, as directed on the package.
Think the nutrients in your soil will have diminished with each planting. Chipchopper has the right idea. You could also take out the top couple of inches of soil and add new if you can be bothered.
compost! make it yourself, buy horse manure and rot it down yourself or spent mushroom compost which smells funny but can be put straight on. Some councils who have green bag schemes (garden waste collection) make it into compost and sell it...might be worth enquiring as its good value usually
Just got back from the allotment .. had 4 hours there today. Funnily enough I was looking at my raised beds in my poly tunnel. They are looking very sorry fo them selves. Dried out and the soil is like powder. What I do every year is dig out a few inches off the top. I then turn the soil over in each bed about a spades depth deep. I make a mound of soil at one end of each bed. Chicken pellets are then scattered on the exposed base of the bed. As I then shovel the soil back from the mound I add more pellets, blood and bonemeal as well as some of last years manure. Keep adding soil and then last years compost. As MM says dried seaweed is also added. This absorbs and retains moisture. If you add a good amounts of nutrients then your soil will improve and should give good returns.
You say last years crop was poor .. what was in the bed that failed to procduce.