ok, sounds and looks like you have some issues that are common and easy for you to fix.
There is a science to all of this. an hvac pro in your area really can do a good job of helping you, if you choose that option, but you can do this yourself if you want to ( I did).
so there are a couple of things the folks here will need to know to really help you.
1. how big is your current hvac system... I am guessing 1400 sqft in Cali,, maybe 2 tons, maybe 3 tons. The size is important because larger the system, the more air it will try to push around. The larger the system the larger the return grill (s) have to be!! And the larger the ductwork has to be. if the return and/ or ducts are too small, your system will be choked, costly to run and may wear out the compressor, indoor fan motor etc before they should.
2. design of the duct work. We need pictures or a drawing of how your duct work is run in the house and what its made of. We need to understand the type, sizes and design in order to make sure that the current duct system can provide the proper CFM and that it is delivered to the various room correctly. The proper cfm is also related to register size and how the registers "throw" the air across the room, thus mixing the air.
after all that we can see what can be done about returning air to the system with the bedroom doors closed.