Default Value For Field Not Populating

I'm a newbie.

My Create Table query had this field:

LockdUntil DateTime Not NULL Default '01/01/1900',

The SP that adds a record doesn't populate this field, so the default value is supposed to populate every time (or so I thought). For the first five records it worked fine, but the sixth record has a value of NULL.  How is this possible?

If I open the table in design view, the definition now shows as:

[LockedUntil] [datetime] NULL,

What happened to my original definition? How could it change?