Replace the batteries, they are inexpensive and easy to replace (remove key insert and use it to pry the halves apart as explained and shown in the owners manual - page 28). The Active Style Fobs (they are constantly transmitting) go thru batteries much faster than the old style Fobs. The batteries only last 12-24 months at best. Even your spare Fob is using battery power sitting in a drawer or on a hook somewhere. Who knows how long your Fobs sat in factory stock before manufacture, or the dealer before delivery or how much use they got. If you use remote start frequently, the batteries have even shorter life.
Buy a battery 5-pack of CR2032 batteries, replace both Fob batteries and keep a spare at home and another in your glovebox.