Why does my monthly balance stay the same when raising taxes?

It can take a month or two before any income changes show up in the amount the game displays as your monthly income. Assuming though that it's been a couple of months since you changed the tax law then the problem is likely that the 11 number doesn't mean what you think it does. The number displayed in the vassal tab of the character window is how much they pay each year in taxes. Divide that number by 12 to get the monthly income they provide. That means if they're paying you 11 gold a year in taxes, they're contributing less than 1 gold a month to your treasury.

As you can see from the tooltip in the image above, Bishop Walram's contribution to his liege's treasury of 6.3 gold is calculated by taking his yearly income of 18.2 and multiplying it by the church vassal tax rate of 35%. That means the 6.3 number is the amount he pays in taxes each year, not each month.

Several parts of the CK2 UI don't update immediately. Almost everything that requires vassal calculations take up to a month to settle out. This includes your monthly balance calculations, both in the top-right quick view and the larger demesne overview you can see at the bottom of the title detail pane for any of your titles. You'll notice the starkest failures in updating when you raise or lower your levies: even though your levy expenses are updated on the military tab, your monthly balance calculations do not show the expenses until the levies have been raised for a while.

Your expenses are dependent upon your vassals contributions, which can fluctuate for a lot of reasons. The money side of things seems like the most straightforward, but other calculations experience the same recalculation delay. Other things that experience a similar delay are vassal opinions and troop contributions.

The main upshot from all of this is that major realm-changing events give you some in-game time to react before all hell breaks loose. For example, if you win a holy war or are granted a boon from the Pope for a successful crusade, you could potentially end up with a great many additional holdings and titles. This makes your vassals hate you for being greedy. Even though you can pause the game and give it all away, the game gives you a little leeway incase you need to arrange marriages, invite courtiers etc. etc. and wait for their responses. This makes sense in-game, as the game makes very few concessions from the limitations of the medieval world being simulated; news takes time to travel, your scribes aren't keeping their books with Excel, etc.

The technical reason behind this, I'm sure, is performance. The game simulates a great many things, and to keep from having to update everything all the time, all at once, they delay many recalculations. A few sluggish UI elements are just a casualty of this.

Paradox Interactive doesn't seem to release official patch notes, but if they did, this might have made my top 10 patch notes of all time list:

But what really hits performance are large empires because it increases the amounts of realms (every ruler technically holds his own sub-realm) meaning more evaluations per character has to be made. For instance I discovered late game that huge greek/byzantine empires were slowing down the game because each greek person was evaluating against each other person in the realm "can I castrate?", and this took up like a huge chunk of the performance of the AI. I think 70% of the AI demands were about castrating or blinding someone when I loaded late-game byzantine saves. Luckily this is fixed in the coming 2.4 patch.

_{Source: Groogy, PI developer on CK2 (emphasis added)}