Problem in using Exchange active sync to pull down emails

Problem in using Exchange active sync to pull down emails

I am the admin of megatrend-ltd.com. I am using zoho recently. After I have migrated our email to zoho, I have used the Exchange active sync to pull down Emails to local clients. 

The mailbox has data sync to around december of 2015 and then it stops. I can't pull down old emails.But I can see it though web interface. 

It is a great problem as some of my users need to check old email frequently. Is that a bug of zoho or is it a feature of it?