When you set your Maesters or Builders a task the required resources will be immediately subtracted from your total. If you do not see the task begin at the same time, this is an indication that your device has fallen out of sync with the game.
To alleviate this issue you should close the game completely and then restart the game. Once you have restarted, you should see that the task assigned to your Maesters or Builders has begun, or that all your resources have returned to their true totals so you can try again.
Keep in mind that if your device was out of sync for a while, the true total of your resources may be quite different from what displayed before restarting. However, the refreshed total after restarting will reflect the correct total for your profile.
If you are noticing your device is frequently falling out of sync with the game, you may want to periodically run through the troubleshooting steps.