Just realised that I had my FT817ND set to USB and not DIG, so I was not sending any modulation on WSPR TX! This has now been corrected, so I am hopeful of some spots this evening. This is the second time I have made this mistake. It remains to be seen if this was the reason for no spots earlier.
UPDATE 1820z: Still no 10m spots! Must be conditions. Will leave the rig on 10m this evening to see if things improve.