Comment 32 for bug 1900168

I had similar issue on my HP laptop with elantech trackpad, where trackpad would not be responsive immediately after login. The buttons would still work, external (USB) mouse would also work.

In my case the fix was to remove synaptics driver and let libinput handle trackpad.

    sudo apt remove xserver-xorg-input-synaptics
    sudo apt install xserver-xorg-input-libinput

I also created a symlink for 40-libinput.conf in /etc/X11/xorg.conf.d although it may not be needed (not present in UM 20.04).

    sudo mkdir -p /etc/X11/xorg.conf.d
    sudo ln -s /usr/share/X11/xorg.conf.d/40-libinput.conf /etc/X11/xorg.conf.d/

A reboot was required for changes to take place properly.