Missing leads Zoho CRM

Since we started using Zoho, we have not built in much functionality, and have only used the CRM/ Leads module. From the beginning, random leads have disappeared. Just dropped out and went missing. At first, we thought we had made a mistake. Now, some of our longer term, high-profile leads have disappeared, and no one on the phone seems to be able to do anything but tell us "this can't happen". That's great, but it did - several times. What's up Zoho - what's the bug?