We actually sent an email to them back on 9/27. Here is what we wrote:

Hi there,

We recently noticed that we haven't received a check in a while and logged into your stats system to have a look. It appears that no checks have been issued for any period after April 30 as those amounts show as "unpaid" in the stats admin.

Unfortunately, we looked back further and realized that we have not received amounts that you actually show as paid. I've attached a summary of the amounts due to us and the payments we have received.

Since this is a rather large amount and there seems to be a problem with your payment system, I'm hoping to hear back from you as soon as possible.

Thanks