Heine

Apart from obvious causes (module not enabled, update hook misnamed), there's a actual pitfall here; You cannot use mixed case filenames for modules.

Background

Suppose you have a module Example with Example.module and Example.install as files. Example.install contains your Example_update_6001() function.

Update.php uses the PHP function get_defined_functions to enumerate update hooks. The function returns lowercase functionnames (PHP function names are case insensitive). Drupal, however, does a case-sensitive string compare (strpos) with the module name. These will never match.

Solution

Use all lowercase module names (example.module vs. Example.module, mymodule.module vs. MyModule.module). Alternatively, wait for a bugfix via Issue #200628.