I have a mesh network made of three M9 access points connected to Google Fiber. The house is very small, only 1500 square feet on a single level. Signal was strong when all three access points were on the same level. Then I started using my basement for household activity; I needed better signal downstairs so I moved one access point downstairs (which made signal weaker everywhere except for the one connected directly to the modem) and bought two more from Facebook marketplace.  
 

may main question is why do three of the four nodes insist on connecting to the "main" access point, next to the modem? This isn't the closest access point.  I assumed that a "mesh" system would connect to the strongest signal, and multiply a strong signal. Is there anyway to force a connection between one node and one close by?  One of the four connects to another node so I know it can work, but the other three insist on connection to the main one the furthest away. 
 

how is the chain or net OC connections determined and can to determine which connects to which or is it something completely in control of the access point?