Loose spoke

[CaptainSlow]

I noticed one of my spokes was loose before a ride last week, I tightened it and did 60km. After the ride I inspected it again and it was loose again. Any ideas on how I can stop this?

eta
rear wheel next to valve.

[upsidedownmark]

Tighten a bit far then back it off a smidge - hold the spoke between thumb and forefinger as you do.

You should notice that as you start tightening, the spoke will 'wind-up' - turn with the nipple for a bit, then you'll tighten it properly. What you want to do when you've finished is unwind the amount the spoke wound up, so it's not trying to rotate wrt the nipple.

Sounds a bit odd that it's just one tho.