I am trying to populate a fk. The way I am doing it is that I have a value selected from a list via a field in the parent table, and then have the pk corresponding to it in the parent table looked up and auto entered in the fk field of the child table. It worked for one record, got the right pk to populate the fk, but it's not worked since. What am I doing wrong? 

I think what you're trying to achieve may be best accomplished with the following:

1. Place the fk field on the parent layout.

2. Define your value list:

    - based on the primary key field

    - choose the option "Also display values from the 2nd field"

    - defind the 2nd field to be the "value" that the user chooses

    - select (optional) "show only values from second field"

3. Note that, in this scenario, setting the fk field Control Style as drop down list vs Pop up field will determine whether the 1st field or the 2nd fields in your value list is displayed when it the field is unselected. 

Many thanks, esp. for the last (3) tip which I wasn't aware of. That's the initial way I went about it, which I suppose is also the standard way to go about it. I wonder why my current approach isn't working...

I wonder why my current approach isn't working...

You could post the file here (or a clone) if you want someone to look at it -- or see if you can reproduce the problem in a little test file.