Tab not working on Microsoft Word 2011

I'm not sure what's going on here, but whenever I go to a new line, tab doesn't work. It doesn't work in other lines of the same paragraph either, but if I go back to a previous paragraph, hit tab, delete the tab in that paragraph, and then go down to where I want the tab, it lets me do it. But then the problem will pop up again in the next paragraph.

How can I fix this problem and what is causing it?

After further experimentation, I've found that just clicking into another paragraph and clicking back does the trick, most of the time anyway.

After even more experimentation, I've discovered that by inserting a blank line at the end of the document, I can solve the problem for the most part. It usually only occurs when I'm trying to put a tab on the last line of a document. However, this isn't a perfect solution and I'm still having the issue sometimes.

I experience the problem of tabs not working when making math tests. The problem is very repeatable, and can be replicated in the following simple experiment:

1. Get a new document and set one tab (say a left tab at 3.0 inches).
2. Type "Solve " and insert a formula, say "2x=6".
3. Use the right arrow to exit from the formula.

You may now type any non-tab characters that you wish, but hitting the "tab" key does not insert a (tab) character.

To fix the problem, do any of the following after leaving the formula:

1. Type (spacebar) (left arrow) (tab), OR
2. Paste a previously copied (tab), OR
3. Click to place the cursor at any point not within the formula, and click to place the cursor at the end of the line. Now type (tab).

My theory is that a flag is set to keep the (tab) character from being used within formulas, and that exiting the formula using the right arrow key does not reset the flag, but clicking outside the formula or backspacing does.