Gapping is the most-often cited cure, but comb leakage is a huge problem. Air can leak either through the bottom or between comb teeth. Plastic recessed-reedplate combs can be especially problematic. the leakage happens because the comb and reedplate aren't flat. None of them are perfectly flat.
The easiest remedy is torque the screws. I came up with this procedure based on what I'd learned replacing heads on automobile engines. If you just willy-nilly tighten screws, or overtighten screws on an engine head, the head won't sit flat and you'll have antifreeze coming out the exhaust pipe. I thought the same kind of torque procedure would help on harmonicas and it does.
I made this video a while back to illustrate it. There's a tag at the end for elkriverharmonicas.com, but I don't sell stock harps and tools anymore.