I had delicious Udon soup at a Japanese restaurant, and my attempts to recreate have failed. what am I missing?
For ramen, udon, and soba, it is not uncommon for Japanese restaurants to use multiple broths for layered flavors.

My friend is from Yamagata in Japan and several of her favorite Udon places will make a sturdy broth with dashi as well as pork and chicken stocks. When I make noodles at home, I almost always start with dashi and fortify with chicken or pork stock. While the aroma of a good dashi is strong, often times it isn't solid enough to feel full-bodied. Try adding other stocks/broths and see where that gets you. Use neckbones and feet if you make your own pork stock.