Video splitters take the output for monitor port 1 and mirror it to the remaining outputs. By default, the first output port is the master port that is mirrored to each additional monitor. Make sure that you use the first output port when you use the video splitter. The master port is labeled on the splitter boxes from StarTech.com. On the splitter cables from StarTech.com, the master port is the top port.
If you are having issues with a splitter interpreting the incorrect port as the master port, try the following:
Unplug all of the video connections.
If the splitter is powered, power cycle the splitter.
Plug in all of the video connections again.
Make sure that the first video connection plugged in is the one that you want to be the master port.