Sometimes not all of the attached files arrive with the email to the customer.
I have the same file attached to multiple events.
(the system seems to be renaming most of them.)
Please submit a ticket with access to your site. There must be something setup incorrectly as DT Register is not going to randomly decide when to attach the files or not Also, it does not rename files unless needed... for example, you can't upload a file for Event B if the same filename was already loaded for Event A.