servers not able to access

one of our prospect clients are having a problem.
They are using three servers and connected to One 24 Port 10/100 switch.It is stacked to one 16Port 10/100 Switch. But when the connection taken from 16 port to systems they won’t get access of one server. When it is taken from 24 port, the systems will get access of three server.They are using Cat 5 cable.
What may be the problem.