First check to see if the vent outside damper is opening when you turn on your fan and manually open the damper to ensure it is not stuck or do you burn scented candles to mask the cooking smells.
It may be cost effective to have the old one replaced with a new one or have the damper replaced could be the issue. With good new it may be alright.
Next the ducting and voids would be sealed of gaps and holes where the air would escape prior to exiting your residence.
Finally the exhaust fan would be checked to see if sucks the air out of the residence and if not is may be time for a replacement. In this case the exhaust fan was replaced after the power was turned off then verified with a meter.
Contact Daniel at 587-597-4228 if you have a problem which needs to be addressed which may be like this or another household issue.