Every summer I teach in an international school in the UK. There are always quite a lot of Japanese students, mostly in their twenties but sometimes older.

As it's a study vacation, the emphasis is on communicative fun and games rather than heads-down grammar work (although there is some of that). The Japanese students always participate dutifully, but I get the impression that many of them aren't really enjoying it, and some consider it a bit of waste of time.

Any comments about why this is, and what I could do to please them more?

Yes, the joys of teaching Japanese adults!!!! It can be a rather stressful thing! You need to have a balance of fun/games and the following:

Reading & Writing Exercises
Speaking & Listening Practice
Grammar Reviews

The Japanese generally seem to like to have a very structured class, and depending on the content that means that sometimes, there's not going to be much room for fun. This makes it extremely difficult when you have a class of mixed nationalities.

Also, Japanese people are quite reserved, especially males. It may be hard to judge if they are enjoying your lessons or not. This can be very frustrating. But if they are continuing to come to class and participate, they are probably enjoying it and are getting something out of it.

Good luck with it all!