Hooks related to private messages not working?

[Zahl]

Hi there,

is it possible that there is some feature/function or something else that prevents plugins that hook some private message related stuff from being executed? I checked the general settings for hooks, but there I can just enable or disable hooks completely.
Maybe its just me, but I tried to add a hook for private_insertpm_complete and it just did nothing. First I checked my code again and again, added ebug outputs, and in the end, I just typed in some random stuff that is not valid PHP Code, so I expected to get a "parse error in eval()'d code" when I would try to send a private message, but it still worked, no errors.
Then I tried to hook pmdata_postsave, but also no luck.
And yes, I selected "activate this plugin"
Other hooks work just fine, like reputation_add_complete...

Any help would be appreciated.