On my 2009 Elise the top button (Lotus logo) locks the car. The middle button unlocks it. The third button I never use but I think it is a panic button.
I have had occasional glitches in which the top button would not lock the car. It turned out that I was parked in some location that had some interference.
If it is that, try driving somewhere else and testing again. Also hold the key near the front of the car. On US/Canadian cars it works best on the passenger side.
You might also try replacing the key FOB battery. It might be low and the FOB is misbehaving.
Last edited by David Craig; 05-31-2019 at 12:57 PM.