Sometimes the review reminder is not sent out

Hey,

I installed Photo Reviews a months ago and everything went well, but now sometimes the review reminder emails aren’t sent out.

I made a lot dummy orders with different e-mail addresses and the review reminder is sent out, but sometimes isn’t.

*I set the review remider email’s time to 7 seconds in this testing period.

What should cause this issue?

Thanks a lot,
Csaba Imre