More information is needed. The Bathroom is under a negative pressure
with respect to whatever is feeding the wall cavity. One thing that
could cause this is if they are using the wall cavity as part of a
duct.

Another thing is if there is a room in the house with no return path
such as an outside laundry. If the laundry has a supply guct, any air
that blows into the laundry must then go to the outside. That is like
running an exhaust fan all the time the ac is running.

A sun room connected to the house with only a sliding glass door would
have the same effect if the sliding glass door is closed.

If you only have two returns, make sure all interior doors are undercut
so air from all rooms have a return air path. Make sure all
conditioned rooms open into the main house. Make sure all supply ducts
that are outside the pressure envelope of the house are sealed.

Remember, when it comes to duct leaks, supply leaks suck and return
leaks blow. That means a supply leak acts like an exhaust fan, putting
the house under a negative pressure, which will SUCK air in any
available opening. A return leak acts like a makeup air fan,
pressurizing the house. Any house air leaks will then BLOW out of the
house. A room with no return path acts like a supply duct leak.

You definately have a problem. You need a contractor who understands
air diagnostics. Hope this helps.

Stretch