Troubleshooting gigabit network issue

Hi all,

I am a novice at enterprise level networking, but learning as I go along.

I have installed a Ubiquiti UDM-Pro + switch to upgrade our previous unmanaged rack. Our setup is:

UDMP - USW-24 - cat 5e patch panel - cables to various ports installed in the flooring.

If I plug in an unmanaged gigabit switch to the floor, devices connected get gigabit speeds. But if devices are connected directly to the floor (no additional switch), the speed is only 100mb.

Could someone explain to me why this is, please? If the device, cables, patch panel and switch all support gigabit speed then why do I need to connect another unmanaged (or managed) switch on the other end to get full speed?

Thanks in advance for your knowledge.