I had a similar thing happen to me when i changed my crank seal. Turns out the problem was that a slight sharp edge had built up around the seal bore so that when I pressed in the new seal the sharp edge was tearing and shaving off a small amount of the rubber material of the seal causing the seal to....well not seal. It was a pretty fast rate leak as well. I bought a de-burring tool (handtool)and it worked like a charm. Did the seal that the mechanic put in have any cuts and damage to it's outside sealing surface? Have you run it with the timing cover off to get a look at what's leaking exactly? I posted about this on here where you can see some pictures of my seal and the de-burring tool.

If it's not a cam seal then it's either the crank seal or the oil pump to main seal mentioned above. Good luck!