Duplicant randomly cannot learn Improved Digging I

It seems to be the moral

On the left, you see moral stats. If your dupe is below the needed value, he refuses to learn a new skill with too high morale-expectations.

I got this several times too, its not about morale because my dupies refused to learn their first ability for what they were chosen.

If its about morale then you couldn't select any other skills either, the skills with heart provide extra morale boost too.