Which part of root to unify S with? (Was: What can be missing from a decl-head-opt-subj mother?)

I got a grammar from the Matrix which has argument optionality, and I expect a certain sentence to be parsed but it not. I have one S in the chart; I am not seeing any inflection issues or MC or COORD issues; interactive unification with the root just shows:

Screen Shot 2020-06-08 at 2.05.03 PM

What’s this pointing to?

decl-head-opt-subj-phrase is a kind of sign, not a kind of synsem. What did you try to unify with what?

I was grabbing the S from the chart and dropping in here:

What should I be doing to check why some S doesn’t unify with the root?

Try dropping it on the sign – the resulting unification error should be more informative!