Why does my beef stew taste "blah" ?

I just cannot seem to get a good flavor from my beef stew. I usually cook it in a pressure cooker or crock pot. I don't like to use salt if I can avoid it--but I include celery, onions (browned in EVOO), balsamic vinegar, low sodium beef broth, canned tomatoes, carrots, small whole potatoes with skin left on and any other vegetables I have on hand. I also add chopped curly parsley, fresh ground pepper and some salt-substitute herb mix that I like. I finally found a good source of meat so at least now the beef isn't tough, but it still lacked something when I cooked it yesterday! Any suggestions?