Web Form Submissions Disappearing

Hello,

I have a web form that I'm using to collect information, and it has been brought to my attention that a number of rows that were submitted by users, and for which they received confirmation emails, never showed up on the sheet. This poses a huge problem as each submission represents a request to process a payment, so we're missing payment requests and don't know anything about them until the client comes back to us to ask why nothing has been done. Does anyone have any ideas as to what might be causing the problem? I've been unable to replicate the error, whatever it is, so far. Any assistance would be greatly appreciated.

Thanks,

Anna