I had the same problem last week. My trunk/boot wouldn't "catch" when I closed it. This happened during the day, when it was very hot (around 29 deg. C), so I doubt if it's related to cold weather.
This is what I did:
1. Tried to lock and unlock the boot/trunk using the remote control,more than 10 times. No luck.
2. Tried to lock and unlock the boot/trunk using the "boot/trunk release" button, more than 10 times. No luck.
3. Locked and unlocked the whole car, several times. No luck.
4. Started the car with the keyfob (I have Comfort Access), to see if this would reset something. No luck still.
5. Manually forced the latch to lock while the boot/trunk was open. Now, the boot couldn't close at all, as it got blocked.
6. Locked an unlocked the car. This unlocked the boot, but it couldn't catch still. I was back to the original problem :mad: .
7. At this time, I was about to drive to the BMW dealership because it was about 300 meters away. As a last resort, I pressed the "boot release" button for about 10 seconds. After that, I tried to close the boot, and it closed perfectly
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NOTE:
- I'm not sure if this was just a co-incidence or the 10 or more seconds press did reset something 
.
- I have booked my car in for CIP 20 update on Thursday to solve about 8 issues that I have experienced since taking delivery.... and when I reported this problem, the Service Advisor said it will be sorted out by the software update. He seemed sure about it. It's a pity that I can't test this afterwards.