I recently discovered that if Windows 7’s “Activate a window by hovering over it with the mouse” feature is enabled, it can affect the ability to insert symbols using the Symbol dropdown (sometimes) and Symbol dialog box (always) in Word 2007 and Word 2010.
So, if you have activated this setting and suddenly notice that you can no longer insert Symbols, this is probably the reason.
One work-around is to use the keyboard to navigate the Symbol dialog. Use the arrow keys to move around, and use Enter or Alt+I to insert the selected symbol.
When using the Symbol dropdown, click the symbol you want to insert, and then click back in the document to complete the action (if the symbol doesn’t appear immediately when you click on it). I find that I encounter this issue less often than when using the Symbol dialog box.
My own work-around was to abandon the feature. I found that it was more annoying than useful. But, YMMV (your mileage may vary).