MSC culture stops expanding? - (Jun/07/2011 )

I have rat mesenchymal stem cells derived from bone marrow in culture. They expand very well, but lately they have randomly stopped expanding. I am using the same culture media and split ratio as always, but sometimes after a split the cells just don't grow. They are all alive, and morphology looks correct, but they can sit in culture for greater than 2 weeks and never divide.
This has happened at various passage number, it has happened at pass 5, 10, and recently transduced cells (GFP+) at pass 23 stopped expanding.
Does anyone have experience with this? What could be causing it, and can I kick start them again?
Thanks!

They may have reached a Hayflick limit - in which case there isn't much you can do (that I am aware of) to rectify the problem.