Access database losing default values for table columns

I have a problem with an Access database that feeds the website of one of my
clients.

In the database there is a members table that has several columns that were
originally defined to have default values, eg. 0 for numeric columns or
Now() for date/time columns.
But somehow these default values get lost while using the webapplication,
although there are no SQL statements that alter the database structure to be
found in the application.

Does anyone here have an idea what might cause this?
Thanks in advance.

Regards, Marja