First thing upon waking in the morning or after a nap is a KEY time for foot tickling. Lazarus takes advantage of the fact my feet are super warm, soft, and sensitive after being wrapped tightly in our comforter for several hours, all the time. I got in some payback this morning with great results 🙂
Also, with some people, they often do not get fully ticklish until they are tired. Then their response time is slower, they lose some ability to 'hold it in', and they are more relaxed, making them more sensitive. If you have a lee who is not ticklish 'all the time', or is gifted with the ability to hold it in quite well, try taking them by surprise just before bedtime 🙂
Mimi