If you have an attribute with 4 options, site works as expected (allowing 4 adjustments). Add a 2nd attribute with no options (ie user text input) and all adjustments options vanish.

Think of the situation selling customized teeshirts, you need a text field for the custom text, but also a size option to track stock (with different SKU for each size).

Workaround :

Create a SKU adjustment before adding the 2nd attribute, its still accessible via the stock tab (if you have it configured).