Note that the dbDelta() function is really picky about how you format the CREATE string with whitespace and line breaks.

* You must have two spaces between the words PRIMARY KEY and the definition of your primary key.
* You must use the key word KEY rather than its synonym INDEX and you must include at least one KEY.
* You must not use any apostrophes or backticks around field names.

http://codex.wordpress.org/Creating_Tables_with_Plugins#Creating_or_Updating_the_Table