I am a low hours pilot, not an instructor but having had a similar issue so I will just describe what the issue with me was...
I think there were a few reasons but for me the main one was that I thought the quality of my flying was really poor so I didn't trust myself. This lead to me questioning everything I did which made things worse. I remember saying to my instructor something like "its not that hard is it?" to myself and that was me trying to work out why I kept stuffing up. EG: Inbound I would forget to say my current height. I would be kicking myself over and over because it sounds like such a simple thing. If I am worring about that so much then how can I be focused on my flying which then leads to more errors...
Oh another was I had periods where I was tired, mainly because of work. Not much that can be done about that except as a human factors lesson!
So, for me at the time the main things to try to achieve were:
1. To relax. EG: being positive, allow time for enjoying the flight, sometimes a prompt to prepare before I needed to do something (like mentioning "we need an inbound call in a couple of minutes")
2. Make sure that they know they are doing well, given their experience. Only when they are of course :-)
3. For minor errors make sure they know of the issue but that it is minor. I think it is also useful to sometimes let them know what they should do if they make the mistake. EG: I had something like "Good inbound call however you didn't mention your height" I spent some time wondering if I should repeat the whole call with the height, just state the height in a second call or not do anything because if someone wanted to know they could ask...
I think I only needed this a couple of times before I got a bit more confident, it also helped teach me what a minor issue was and some corrective actions so if I did it again I didn't quietly stress about it so much.
Sometimes the reason they aren't relaxed (and able to focus) is because of non-aviation related factors so there might not be much you can do about it except to let them know that the reason they are having problems is because they are not relaxed. When back on the ground it can be a good real-world lesson for the student on human factors :-)
I seem to remember on one occasion where I was really not able to perform my instructor just said I seemed really tight so how about we just cruise a bit and enjoy the scenery. That lesson because S&L with some turning around a point and other bits disguised as sight-seeing :-)
After a flight it can be good to discuss good and not-so-good aspects of the flight. Especially negative easpects, the stress of the flying makes it easier to think and talk :-)
Steven.