Coating the new o-ring with coolant is pretty important. It not only keeps the rubber from drying out, but also aids in the installation. If you didn't do this, you may have pinched the o-ring, creating a tiny gap. Or the o-ring may just be defective. Either way, you'll have to back out the water pump to see what's going on in there. You did use a new o-ring right? (just checking). And don't use any type of gasket compound. If you do, you'll strip the M6 bolt holes trying to get the pump off in the future. A properly working o-ring is more than sufficient to create the seal.