Using Autonumber as PK, cascade update doesn't work?

Greetings all,

I've been searching and searching and trying different things and I finally
need to ask.

Is it possible to use an Autonumber PK to cascade update new records?

For example, I have three tables, let's call them Main, Sub1 and Sub2:

In Main I have an Autonumber PK field named MainID.
In Sub1 I have an Autonumber PK field named Sub1ID, and a Number field MainID.
In Sub2 I have an Autonumber PK field named Sub2ID, and a Number field MainID.

I have set up a one-to-many relationship between Main and Sub1, using the
MainID field. Ref. Integrity Enforced, with Cascade Updates checked.

I have set up a one-to-one relationship between Main and Sub2, using the
MainID field. Ref. Integrity Enforced, with Cascade Updates checked.

So when I enter a new record in the Main table, I want that MainID
Autonumber to cascade update into the MainID fields of Sub1 and Sub2. I
don't know if it's not working because it's a new record and Cascade Update
only works if you *change* an already existing record, or if it doesn't work
because it doesn't like my Autonumber.

If anyone has the key to this I would be eternally greatful. I've tried
everything I can think of on my own, and from browsing here, Access Help, etc.

Thanks very much!