It sounds like you had enough spices. Are you sure you used meat meant for soup? Cuts intended for frying/roasting are more tender, but have much less aroma. For soup, you need older, tougher meat. Also, you say "Cubed beef", which sounds as if the bones have been removed. Cooking the meat together with the bones gives you more flavour. Third, you are supposed to sear the meat and sweat the onions before simmering, because that develops flavour. It doesn't get clear from your question if you did that.
Another way to have more flavour is to use more fat. Even if the fat itself is neutral tasting, it makes you taste the other aromas stronger. Add it when sweating the onions.
It may be worth trying to first roast the barley in the oven, or even prefrying it, like prefrying rice for a Balkan/Middle Eastern dish. I haven't tried this and cannot guarantee that it will work.
You can also use MSG or other glutamates. They are usually not sold pure in the supermarkets, but are included in other products, like broth cubes and seasoning powders. Maybe you can get them pure in a Chinese shop.
None of the ideas above apply to your already cooked batch soup (you could take out some of the liquid, dissolve a broth cube, and put it back, but it will make it much saltier). The only things I can think of will add flavour, not strengthen it, thus changing the original idea. I don't know if this is a problem for you.
If you don't mind changing the flavour, you can 1) add other herbs (they need to be fresh, dried ones should have cooked a bit). Parsley, marjoram, and lovage come to mind. 2) Add condiments. Worceshire sauce is unusual in soup, but works well for my taste. 3) Cook chopped dried mushrooms in a very small amount of liquid, then add them to the soup together with the liquid. 4) throw in crumbled feta cheese (good quality, preferably from sheep milk).
Other ideas which change the flavour, but cannot be applied to the already cooked soup, would be adding celery and bay leaf.