Mastering Artisan cheese has a nice rundown on what's happening inside of brie and camembert. The short of it is that when the culture inside the paste begins to ripen - it produces lactic acid. The target ph for brie is around 4.4 to 4.6 in the recipes that I use. Once the lactic acid begins to be produced - the p. candidum feeds on the lactic acid and causes the ph to become more alkaline. I'm not exactly clear on this (I will have to go back and look) but when the ph is acidic, the cheese ceases to be "water loving" and would be on the dry side. When it becomes alkaline - it starts to be "water loving" again - which is called "resolubility." This is how it becomes gooey.
As for bitterness... I've been making a lot of giant brie's and mine have been (I think) due to too much rennet or too much moisture when I've drained them.
If you post your recipe, a lot of great guys on here can analyze it.
Sam