Some gas leakage is expected, what you have is what most consider normal. If you have some true 5.56 ammo, your gun will probably operate since it runs at a higher pressure. But most all 556 ammo is tamed down some.
If your gas port is in fact 0.078" that is the min spec. This could be your problem. The spec range is 0.078-0.081" . The last barrel I made/turned and fitted, the min spec hole did not give enough gas to operate the bolt, just like your having. Drilling the hole out to mid spec corrected the problem. This is best done with number drills and/or reamers. I set mine up in my mill to drill it out. If you do this run a brass rod down the barrel to prevent from damaging it when you go through. Then you have to clean out the chips and any flashing still stuck to the barrel. A good fitting jag and patches works fine just takes several runs.