I assume that you only get a picture on your external display, but the usb peripherals do not work?
You’ll not get docked mode if you only connect a monitor even though the external screen is working - just tested it on my L5. Even if I then connect a bluetooth keyboard docked mode is not entered and the on-screen keyboard is still shown when keyboard entries are expected (this I’d consider an issue).
You should have a look at the output of sudo lsusb and journalctl -b and compare the information about usb devices between an occasion when the docking happened as expected and one where it didn’t work.
Forcing the docked mode somehow will probably not help to make your usb devices work. If you’d connect your usb keyboard or usb mouse without connecting to an external monitor your L5 wouldn’t enter docked mode neither, but your peripherals would work well (and you’d be annoyed by the fact that the on-screen keyboard shows up even though you’re typing on an external keyboard )