Change its batteries.
/serious
Transform to Ram and it will be fixed.Same happened with the DuskByte +5 set I gave to Grim,he transed to Surge and the arm wasn't glowing and I was like "WTF I'LL KILL YOU IF IT'S BROKEN",then he returned back to Ram and it started glowing again.He gave me back the set and everything's ok unless you are Surge.