I have just bought a 3 pack of deco m4r's

Main unit (1)connected to router via ethernet 

2nd unit in kitchen (2) connected to (1) wirelessly

3rd unit on first floor (3) connected to (1) wirelessly

All units connected to the Internet and showing steady white led 

I can stand next the kitchen unit (2) and the signal to the main unit drops to 1 bar with extremely slow Internet yet it won't connect to the m4r I'm stood next to! 

What's the point of the mesh if it doesn't relinquish connection to the main unit when signal strength is so low? 

Suggestions welcome. If I can't fix it I'm taking it back.