Could also be that the Bluetooth kernel module is a loadable one, and if you’ve updated the kernel (which will usually take place pretty soon after a first install) then you won’t have the matching folder of modules to load up until you restart. Arch is a bugger for this - I’ve an external mouse that works fine if you keep it plugged in during a kernel update, but it won’t be recognised after an update until you restart again. Not a big deal - you can choose when to update.