When trying to add row or columns it says that I have two overlapping tiles and won't add anymore rows or columns

That means there are some tiles that are stacked on top of each other. If you go through and drag any of your tiles you will see find which ones are overlapping duplicates. Once there are no tiles stacked then you will be able to add columns and rows.

Watch this tutorial to learn more on how to edit your webmix:

Feedback and Knowledge Base