Devices not transitioning to extended airport

Hello Apple Core,

In my house, I have an airport express setup next to my modem (upstairs), and an airport extreme (downstairs) where most wireless activity is used. Both devices are of the latest generation. I have set it up as follows; the airport express was first, I created a wireless network, secured, etc. Then I setup the extreme to extend my wireless network via Ethernet (I'm using nice power line adapters for the Ethernet). These were setup with the airport utility on my iPad.

My issue is that my WiFi devices are not transitioning from one router to the other. For example, I can take my iPad mini or Nexus 5 upstairs and connect to WiFi, and looking at the MAC addresses that are accessing the routers, I can see they are properly feeding off my express. However, I'll take a trip downstairs, look again at my two devices, and they are still feeding off my express rather than the extreme which is within arms length. It really hurts too, because after performing a speed test (speed of.me), I'll see there's almost a 20Mb/s drop in download speed if they're not transitioning. Once I drop WiFi and reconnect to my network on the devices, I'll see that they properly go onto the extreme.

Any advice? Thank you!