Does your NFC antenna works half the time? Disconnecting during the night leaving your phone with empty batteries in the morning? Don't be sad, your NFC antenna may just have become loose.
The antenna is located in the back cover and connects to the phone with a little metal part with spring in it. This ensure that both metals stay connected all the time, but apparently this fails over time.
The temporary fix
To fix the problem RIGHT NOW, you could just put some pressure with your thumb on the back cover where the connections resides (you will possibly ear some click sounds).
...but you'll have to repeat this process from time to time and in about 3 weeks it won't work anymore because you'll just have increased the looseness of the connectors...
The "permanent" fix !
To fix the problem more permanently, remove the back cover like this :
then unglue the connector from the cover and put some layers of masking tape under it (about 4 or 5 layers should be enough).
This will raise the cover's connectors a bit so they always touches the phone's connectors.
Close the case back and that's it ! The antenna should be working fine now.
If the problem returns, put more layers of masking tape !