Frozen veggies meets gumbo = flavorless. How to add flavor?
I think the problem is that in the original recipe you would have browned the veggies in the roux, which develops flavor. Since you didn't do that, you might want to saute some onions and garlic until well browned and add that in. Other possibilities:

(1) Sauce may just need to reduce and become more concentrated
(2) May need more salt
(3) May need a little more acid (a bit of vinegar or lemon juice)