Question: I'm using Word 2010, and I'm working on a guide where contents to be added inside a table of three cells.
I have an issue with couple of pages where I couldn't insert the cursor into the left cell to add a text. It works in some of the pages, but not for the others.

I checked the Developer tab to see if any protection is active but nothing. I tried also to copy those tables into a new word, but got same result.

I couldn't find an answer online. So I appreciate your assistance.

Answer: 
I had this problem as well. It was caused by the cell margins being set larger than the cell width. Try going to the Table Properties, then the Cell tab and select Options. Then reduce the cell margins.
Adding an additional column in a Word table caused no left/right margin in the cell.
I highlighted the cell, I went to Table Tools>Layout>Cell Size
Select "Cell" and Options. Put in the cell margins as preferred e.g. 0.19