Have a REAL plumber install an indivual vent as the better codes require. Try to picture a 2 liter bottle of water held up side down. The water goes plop plop plop starving for air. Take this same bottle and put a hole on the opposite side and WHOOOSH GREAT flow as the atmospheric pressure equalizes and allows full flow. THIS is called VENTING and without a proper vent it isnt going to ever work properly EVEN if you opt for over sizing the drain size. DO NOT use an AAV as why risk your family health on a piece of rubber?
If you have a common vent, each fixture has their own vent in another words. You may need to have your vent line water jetted to get rid of possible scale and or soap scum build up. Have fun