Problems With Relationships Showing Up In A Form

I'm designing a relatively simple access database with four linked tables:

- users
- computers
- licenses
- license_users

Each user as a computer_id linking to the computers table; they also have a set of rows in license_users, linked via their userid, that join them to the licenses table. The relationships are enforced with referential integrity. Everything works fine when I enter data manually in the table datasheet view.

I've designed a nice form to make it easier to enter data for all the tables at once, but when I try to enter the data it all goes wrong. I'm unable to enter any data into the users table fields until I have complete the computer and license data. If I enter that first, then start entering data into the users fields, the computer data suddenly disappears. If I inspect the table then the computers and license_users has been entered, but nothing is entered for the user and I can't get the rows to relate to the users!

It's a while since I designed any Access databases, and I'm stuck, so suggestions would be appreciated.