Why is hook_update_N not running?
Another reason why an update hook would not run is a mismatch between the update hook name and the major release number of the module (see .info file).

For example: the update hook mymodule_update_7001 does not run if the module version is 7.x.1.0. In such case you must rename the hook to mymodule_update_7101.

See the hook_update API documentation