A theory of don't

I realize that I'm a "therapeutic" blogger - trying to make some sense of what's happening in the daily life of a junior manager. So do expect some ranting and nagging again :-)

Today, my dear audience, is the theory of "don'ts" - Why does someone refuse a task ?
A wise manager - who happens to be an avid reader of my posts - had the following theory:
A person is resisting a task because either:
1) he doesn't understand it
2) he doesn't have time for it
3) there is something 'political' going on.

Today one of my reports just plainly refused to do something.
A bit of context: Last december, she had been assigned a different set of tasks due to an internal re-organisation. As she felt demotivated by this change, we agreed that she could move to another position within the company (job rotation is very common with us). Being rather selective on finding/accepting a new position (no change of location, outlook to promotion, more soft-skill oriented), it took her a bit (too) long and she has suddenly been forced into a new role on Monday by HR.
However, to ensure a smooth transition and exit from my dept, I  made arrangements with mgmt that she will still work for me one day a week for the coming month.
When I spoke with her today on this, she bluntly replied that she won't do it, bla-bla-bla..... Euh, wablief,... who's the boss, what about professionalism ? ....Time for the "John Biermann" methodology.

So here is reason number 4 for the theory of dont's:
4) people start taking too much for granted.

To sleep now.