Do both tables have defined primary keys and using autonumbers? I mean formally defined as such in Access? It would be impossible for the table adapter to know what filter to give the select if it can't tell which column is the primary key and what is the next value of the newly inserted autonumber.

I'm not sure if that helps, but I've had to work with poorly designed Access databases and I had a few options disabled like this because sometimes, the primary key was just a normal autonumber without a primary key status within Access and the designer couldn't tell it was the primary key. It may be something similar that is happening, like you forgot to set a property on a field somewhere...

Just an idea, if there are more than one autonumber on the same table (which is not allowed on SQL Server, but I think it is in Access), it may be the reason why the reason why the designer cannot tell which one of the two autonumber sequence to use for telling what is the newly added primary key.

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The human mind's capability to comprehend abstract concepts is limited to the vocabulary it can use to describe it. The more precise the building blocks, the more complex the thoughts that can emerge... Sounds like the evolution of programming languages doesn't it?