Fork oil change

I have a friend who has an '07 TC 510. He is in the process of changing his fork oil and can't get the action of the forks to work correctly. On rebound the forks snap back pretty hard. Seems like the oil is not being drawn up into the chamber. Does anyone have any advise for changing the oil on these forks? I have a TE 510 and I had none of these issues when changing the fork oil. Any help would be appreciated.