I've been experiencing a problem which I cannot figure out the cause. I have a device(A) connecting to a switch (S) then a router. If I connect the device directly from a laptop(B), am able to reach the device. 

A to B = works

if I connect A to the switch and B to the switch 

A to B through S = Not reachable

If I introduce another computer(C) and assign it a similar IP(that of device A) 

C to B through S = works.

Device A has no firewall the subnet is the same for all the devices (255.255.255.0) so is the IPs (192.168.0.*).

At first I thought the subject was the problem but C connects to B thro the same setup. or is the device ginxed for real.

Help