max 485 master and slaves problems
Make sure that different slaves have different Modbus addresses.
You have strong biasing resistors (470R), they should not be present on every device. Remove R16, R25, R23 and try. Most of the time the bus will work fine without them.
Remove termination resistors from every but the first and last device on the bus. At low speed (eg. 9600bps) the bus will work even without them.
Try multiple slaves at lower speed (eg. 9600bps). If it works then you have a signal integrity problem.
Test with a known good implementation slave (eg. FreeModbus).
Get a USB&lt;->RS485 converter and test with a known good master implementation on the PC (eg. QModMaster).