Hate to admit it but it has happend to me. Sometimes when things just get a tiny bit soft a person (like me) might start to worry it is going to get worse, and that is a sure recepie for it to get worse and worse.

On the plus side if you are understanding and don't put any undo pressure on him, it is less likely to happen, and personally, after I let go of my stress and relax with my wife still next to me, often things will start to work again, some of our best nights have not started off well, but ended GREAT.