Hi Melsyo,
according to your PSU specs you should have
PCI-Express Connector
1 x 6-Pin, 1 x 6+2-Pin
your r9 290 should have
1 x 6-Pin socket and 1 x8-Pin socket ?
where is the problem ? are you sure you are using the right cables ?
the 8 pin will be the 6+2 pcie cable to make the 8pin