This is one of those hardware questions that's making me bang my head against a wall.
About six weeks ago I spent a day tuning the NFC antenna on my board. I ended up using ~71pF of shunt capacitance (a 24pF and a 47pF in parallel). I tried 24/47pf configuration on a fresh board and, lo and behold, it worked! So I put it aside. Now, coming back to it, I've put the same values on yet another fresh board and it's not working.
Could it be the manufacturer? The components I'm using now are from a different manufacturer than the ones I used before. I've heard that this stuff is sensitive enough that even sourcing components of the same value from a different manufacturer can mess up your design. Or could it be some ambient factor like temperature or humidity? It's gotten a lot warmer here over the last few weeks. I'm having trouble believing that it's the board. These are all boards from the same manufacturing run, and should be more or less identical.
How can I tune the NFC antenna reliably?
This is the RF chip I'm using.
And here is the application note for tuning its antenna (including a circuit diagram).
The old caps I was using (I think - I tossed the packaging when I finished them, but the other caps in the tuning box were Murata):