Underassertive

This problem multiplies even more as Hart and Blackmon report: “Clinical experience shows that underassertive and excessively overassertive ministers are much more likely to suffer from depression, burnout, helplessness and especially problems with anger and resentment."

I once heard someone offer an underassertive friend a favor and the friend said no in such a roundabout fashion that the person actually thought she'd said yes, and did it anyway.

Some people are underassertive in almost every kind of social situation, whereas others behave overassertively in social encounters.