When ethernet was young, it was via coaxial cable, which only has one signal wire, and a ground. this only allows one device at a time to be sending, if more than one tries, we have a collision. In case of collision, both devices wait a random time and try again, ideally at different times.
With the advent of 10BaseT, each device has a transmit pair and a receive pair, so it COULD both transmit and receive, the the intertia of the 'way things are done' was not changed quickly. Hubs transmit every packet to every port, so one cannot receive while transmitting. (half duplex) Switches were invented to send packets to just the ports that need to receive them, that allows a port to be both receiving and transmitting. (full duplex) Few 10baseT devices were made with full duplex in mind, by the time that switches were in wide use ethernet had moved to 100 megabits.
Soon after 100baseTX was deployed, 10/100 switches started to become common, one early model was the Synoptics 28115. As mixed networks of switches and hubs; 10 and 100 Mbps became common, protocols were developed to auto sense the speed and duplex of devices. The 28115 does not autosense either speed or duplex. While modern devices can auto sense speed even with really old equipment, the autosensing protocol for duplex requires both sides to use the protocol to work.
I set up my 28115s as 1-4 10 meg 5-8 100 half duplex and 9-16 100 full duplex, and start plugging in at the 100 end. if a device will not link at 100, it moves down to the 10 meg ports. as I find the actual best speeds of my devices, I adjust how many 10 and 100s I need.
Because the 28115 will not autonegotiate, you need to set the speed and duplex of your other 100 meg devices to a fixed speed and duplex
On a Windows PC, this is most commonly found at START settings CONTROL PANELS networks ADAPTORS properties, although some cards add their own control panel and some add an icon in the Systray in the lower right. try for 100 full, but 100 half ifs better than not being able to set it at all.
if you find you cannot set it at the PC, try copying files both to and from the PC at the same time, if half is faster than full, set the 28115 to half.
Now for the hardest part, if you move these PCs back to a modern switch, you need to set these PCs back to auto negotiate. Go ahead, just try to remember to do that!