I've had some issues with KDE Plasma 5 detecting a Wacom Intuos4. I tried several solutions and finally got it to work normally by NOT installing input-wacom-dkms and installing xorg-xinput along kcm-wacomtablet, libwacom and xf86-input-wacom (that were already installed with KDE).
At one point I had installed both input-wacom-dkms and xorg-input and didn't work. The trick was uninstalling input-wacom-dkms and installing xorg-input only.
I thought I'd post it just for the record hoping it might be of some help to somebody else.