If you're using cumin powder, you could try switching to cumin seed and then grinding it right before adding it. This might make it more flavourful. I might also add, for the rest of the ingredients, garlic/garlic powder and maybe onion powder as well. I don't know how much salt you add, but just increasing the salt by a bit, might bring out the rest of the flavours.

I often serve something similar, except I use shredded beef, the meat is different, but the seasonings are fairly similar. [The recipe I started from][1] calls for the following. I'm posting it as it gives an idea of the quantities used. For about 2 lbs of meat:

 - 1 level tbsp. salt (I actually reduced this as it was a bit excessive the first time I  made it)
 - 1 tbsp. pepper
 - 1 tbsp. cumin
 - 1 1/2 tbsp. chili powder ( light or dark )
 - 1 tbsp. garlic powder (I usually use minced garlic)
 - 1 tbsp. onion powder (I omit it due to a severe allergy that my wife has)
 - 4 bay leaves (It's stewing for hours to break down the meat)
 - Onion (added mid cooking, for shredded beef, I'd add it prior to cooking if you're using ground beef. Omitted for me, for the same reason as the powder)

I have slightly increased other ingredients, to compensate for a lack of onions. I may try using some vinegar next time.

  [1]: http://onestopcook.wordpress.com/2008/01/10/authentic-mexican-style-shredded-beef-tacos/