Unless you have a high powered processor that is overclocked, or have multiple graphics cards, an extra eps connector may not be required at all.
Look at the pin arrangements in the socket and on the plug.
There will be an arrangement of square and partially rounded holes that should match up.
Do not try to force anything.
Orient the cable so that the latch on the cable is on the same side as the notch on the socket.
What is the make/model of your psu?
What is the make/model of your motherboard?
What is your cooler? Yes, it may be a pain to remove and remount.
Do you see a unused psu cable labelled 12v eps? Unless your psu has 2 such cables, you may be using an incompatible cable.
If the cable you used is of the 6+2 variety, it is undoubtedly a pcie cable, not an eps cable.