Sounds like the side switch is working since it can still go into the strobe and SOS. Either the driver is defective, or the batteries are not properly charged.
could be a lot of simple things. Fenix should take care of you they sure as heck took care of me. They rebuilt a waaaaay out of warranty PD35 for me for free I couldnt believe it. They will ask you for the lights serial number find it write it down.