Can’t add shape hints

It appears that I can't add shape hints to a certain layer or to new layers. But I can add shape hints to other existing layers. This is really strange. Why can't I add them toa shape tween in a newly created layer? And why can't I ad to that one specific old layer? It just stopped working. I could add to that layer before but not now. Or possibly the hint is added but the marker isn't visible or something like that.

please help!