First of all, if you are using a compositing manager, make sure it unredirects
fullscreen windows. Also make sure vblank_mode isn't set to 0 in ~/.drirc or
/etc/drirc or as an environment variable.

With that ruled out, we'll need to work backwards why the driver can't use page
flipping. The final decision on that happens in the X driver's can_flip
function in radeon_dri2.c. From a remote shell, attach gdb to the X server
process and set a breakpoint on that function. If it gets called, check which
of its conditions are not met.