Like any mechanical or electronic device, over time, a key fob will become worn and no longer function properly. This could be caused by any number of things, but usually it is because a contact within the key fob has become worn down and needs to be replaced. It could be because debris has been caught under the particular key fob that isn't working, disabling the electric relay that sends the signal for your boot to open.
Make sure that your boot lock is operating properly. Do this by opening it from the inside of the car. Make sure it is lubricated properly with penetrating lubricant. Test it a few times to eliminate the possibility that it is your fob.
Reprogram your key fob. Place the key in the ignition and turn it from "Off" to "Run" eight times in 10 seconds. Stop at "Run." Listen for the doors to lock and unlock then hit a button on the key fob to program it. The locks will lock and unlock again. Turn the key in the ignition to "Off." Test the key fob to see if it will now open the boot.
Open your key fob by using a small slotted screwdriver to pry between the two halves of the fob. Once it pops open, gently lay both halves down.
Blow the transponder circuit board clean by using your canned air. Take out the buttons, and make sure that the area around them is clean as well.
Replace the battery by putting the positive side up. This will eliminate the chances that yours is sending a weakened signal. Close the remote and test.
Clean the contacts if the fob still does not work. Re-open the fob, and using a safety swab and a isopropyl alcohol, clean the contacts by gently rubbing them. Allow them to dry. Then close the remote and test again.
If your key fob still will not open the boot, consider taking it into the dealership. You also can consult a reputable car locksmith for your fob problem. Use a professional organisation to find a locksmith you can rely on.