You probably have a leak either along the lid gasket or the stopper where it plugs into the lid.
Those are really the only two paths for CO2 to vent out of the fermenter.
Just like you can test an inner tube for a leak with water, you can do the same here: just spray a little water (or sanitizer if you want) from a spray bottle on any suspect area(s) and try to narrow down what is happening.
You may have a cut / damaged stopper for example. Also, your gasket may not be seated evenly in the lid channel. In a rare event you could have a clamp bent somehow and not engaging firmly on that section of the lid, then you may need to tweak a clamp tighter.
We have an FAQ for that here:
IF YOU HAVE FTSs, this is another path for CO2 to escape: loose nuts on the lid coil(s)!!