Recently, the key fob for my 2014 Nissan Note has been able to unlock all the doors except the boot. Please advise on what the possible fault might be.

HI Kelvin

Have you changed the battery on the keyfob recently?

I think possibly the same as when you take the fob apart it is a known fault on some of the fobs for the lock/unlock buttons on the fob (*which can be separate to the boot lock) become detached from the circuit board, (*depending on which fob you have).
You can repair the fob with a kit from eBay and as long as you have soldering skills and a steady hand then its a very cheap job to do

https://www.ebay.co.uk/sch/i.html?_nkw=2014+nissan+note+fob+repair+kit+microswitches&_sacat=0&_from=R40&_trksid=p2334524.m570.l1313&_odkw=2014+nissan+note+fob+repair+kit&_osacat=0