Weekly report sending stopped working (2 replies)

I have installed Piwik 2.12.1 on my server and check about 70 websites with it.
Some of the user asked me why the automatically report send function doesn't work anymore.
If I click the button to send report manually, I receive the mail as usual, but the system doesn't do it
on schedule base automatically.
I run PHP 5.3.3.

Any idea where to check it?

Thanks in advance.

Maik