Child workflow start delayed

Hi,

I have a Parent workflow wich starts a Child worfklow during execution.

Everything worked well during the last 2 weeks except yesterday :

- Parent workflow started on 5 items at 6 PM

- Child worklow seemed pending

In fact, all 5 Child workflows started....between 9 an 11 PM !

Is there any reason for such a delay ? Generally, it was about 3 minutes....