Why is hook_update_N not running? I am porting a module to Drupal 7. The module has hook_update_N() functions called from hook_install(). There are some insert queries made inside one of the hook_update_N() functions. But my hook_update_N function is not running. Is there any way to explicitly invoke hook_update_N functions?

I know my question is almost same as this How to get hook_install() to run all hook_update_N()?. But I don't understand how hook_update_N() is called. I have run updates in my Drupal site which I think also run updates of custom modules but it seems hook_update_N() is not getting triggered. Because, I have added error_log() function inside hook_update_N() but the log file is empty.
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