i had just installed an r9 290 after using an 280x.
used DDU for deinstallation of the drivers.
but it not worked.
after finding out, i had to use an Displyport cable
it still didnt worked...
my LG monitor still showed freesync inactive, so i reseted the monitor to default
after this it started working
some notes in the interent some one whroted that to disable blur reducition on benQ monitors
hope it helps you or other ppl