Sometimes ticket can't record on Zoho Desk

Sometimes ticket can't record on Zoho Desk

Hello, When we received questions from our customers by email, questions are recorded as ticket in zoho desk.
But sometimes some questions haven't been recorded since the end of March.

then when our customers resend email for request, the question was recorded.

Why were occurred this situation?
Would you please give us some advises for handling this situation?

Best Regards,
Etsuko Shirotani