https://www.nasa.gov/podcasts/houston-we-have-a-podcast/arti...This podcast covers a lot of the status on international studies for artificial gravity.
"Now we did try to fly a centrifuge on station around 2009 or 2010. And again, it was the same international community and we put together an international proposal to do it on station. And it turns out after a lot of hemming and hawing and the U.S. was going to provide the crew and the integration, the Japanese were going to fly it and the Europeans were going to pay for the device and have it built, and flight certified. It turns out that the station really isn’t designed for that much vibration. So instead of just having a person laying there, we were going to have a person with a cycle ergometer that would spin and do the ergometer. And that much vibration was bad for the station and it was decided that you would have to have a very expensive vibration isolation system that the Europeans weren’t willing to pay for to be able to do it. So, we kind of lost the possibility of doing human centrifuge work on station at this time."
So it looks like this is the current issue, and there is a large amount of work going into the physiological side of research to find out exactly what amounts and methods of artificial gravity achieve desired results.