Let's make sure you're doing the right sequence first off (make sure the fob batteries are fresh):
1. Press the lock button on the remote. The car must have been recently locked by the fob first to remote start. It will not remote start if the car is unlocked or if it's been too long since it was locked from the fob.
2. Quickly press the remote start button on the fob TWICE. A press and hold or a single press won't work. On Ford's its a quick double press.
3. If it works, the green light should flash on the fob. If it fails, it will flash red.
My alternative theories:
1. The car isn't equipped with remote start, but at some point the fob was replaced with one that has the remote start button on it. There are two versions of the fob that year and both are interchangeable other than the remote start feature
2. Check that MyKey isn't enabled for your fob (refer to the owner's manual and the instrument cluster menu)
3. Check that remote start isn't disabled in the menus (refer to the owner's manual and the instrument cluster menu)
4. At some point an aftermarket security device was installed that overrides or disables the factory remote start. You can try and check under the dash for a box that doesn't look OEM, but you may not see it since there are many hiding places for these. If it ever spent time in Canada, this is a common issue as Canadian insurance gives discounts for having a third-party "dead man" magnetic disabler installed in the car.