1) What style is it?
2) When you say not bitter enough, is it because there's too much residual sweetness or has it fermented all the way out and simply lacks bitterness? How long was it in the primary/secondary?
3) Was it an extract batch you did using a partial boil?
4) Can you post the recipe and your process? How much and what yeast did you use? How did you aerate?
Note that carbonation will add carbonic acid which lowers the pH and brightens up a beer.
A good example is to get 2 bottles of plain carbonated water (Talking Rain blue bottle from Costco is perfect for this) and loosen the cap on 1 so it will de-gas. Keep them both in the fridge overnight, and taste them side-by-side to see the exact flavor differences that CO2 imparts into your beer.
The light bitter/sourness balances out some of the sweetness, and the co2 volatizes out some of the hop aromatics which your nose and tongue perceive as overall increased hop presence.
As far as making a hop tea and back-bittering, generally, trying to isomerize the hops at this point in a separate boil will result in too much of a grassy, vegetal flavor and detract from the beer. Morebeer.com sells iso-alpha extract with is literally the bitterness compounds which you can add directly to your beer to rebalance after the fact in cases like yours. Its not too expensive and I use it when I'm teaching BJCP beer judging classes.
Another option, If you have the fermenter space, and you are patient, is to brew another batch of the same style, make it over bittered, then blend the two together before you bottle/keg. Doing that will also allow you to condition this beer longer.
hth
