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.
My table span was over 4 pages with only 1 page being affected. I put the cursor in the column I was having trouble inserting text and right clicked and under Table Properties check the column size. I did this on several pages and realized that the one I could not edit had sizes for the columns on that page, but the other ones were dithered at a whole size of 10.1. Also I preview my pages at 50 % to only focus on the columns and this help me realize when I had clicked the column that had the problem it highlighted the following page. I turned on the paragraph marker and notice I had a hard return at the top of the page with the problem column, so I deleted it and at the bottom of that page I place a page break to move the border row back to the top of the next page. This was definitely one I had seen before.