About 5 times a month I have sewer smell in my house. I had a plumber do a smoke test but no leaks were found. I suspect it is related to the barometric pressure changes... but what can I do to fix it??

It is very possible. Usually, you can smell sewer odor a day or so before it rains. What happens, sewer gas is not rising but rather hangs around the house. If this is it than there is really nothing you can do about it. Just make sure all vents are terminating above the roof.

Milo

Kastelhun,
Roof pitch, overhanging flora, tree lines, topography, prevailing wind, and barometric pressure can all contribute to forcing normal sewer gas venting into a downward flow. There are vent pipe filters ( Google sweetair or odorhog ) that can be installed on the rooftop pipes. These filters are charcoal activated and very effective. When the soil pipe is accessible where it exits the building, you can install a house trap which will prevent odors from entering the dwv system; we always install them with double clean outs, and, in some jurisdictions, they must be installed with a relief vent...

Thanks!

Do you have any floor drains in your basement or bathrooms seldom ever used? Do your plumber check each wax ring on every toilet?

Yes, there is a drain in the floor next to the water heater.
Wouldn't the smoke test of shown a bad wax ring?

Pour a quart of water in the floor drain to replenish the trap, no not necessarily.