The time of a presentation is finite. You have 20 minutes, not more. So, if you discover that you did not address a certain issue, message in your presentation, you can only add it at the expense of other content.
Just "adding a few bullets about it on page 12" will have 2 implications:
- Your extra point will not be made, because it does not feature strongly enough in the presentation
- The power of whatever was already on slide 12 gets diluted by the extra visual clutter or more text
If it is an important point you need to make, make it at the expense of another. If it is not important, don't bother putting it in. Squeezing does not result in great presentation designs.