Experienced LER here:
Ok....I've had this problem (?) for several different female LEES in the past, and I always try to figure it out. Let me try to do this as logically as I can:
-If she is super stressed without any wind-down time before the tickling starts, she may be way less ticklish.
-If she is chilled, cold, or it's a cold day and she hasn't been bundled up..she'll probably be way less ticklish.
-If she is nervous, doesn't know you well, or isn't comfortable with any aspect of what is going on..probably less ticklish.
BUT
-If she is ovulating, or within 3 days or so of it, she will probably be heightened sensetivity and will be MORE ticklish.
-If she is warm, comfortable, and her skin is not sweaty, she will probably be more sensative.
-If in particular, her FEET have been in warm socks and shoes all day...more ticklish!
-If she has used any kind of vibrator or already had sex that day, she may be MORE ticklish already!
Sometimes though, it just depends on the day, and how much they feel surprised by your movements. I've had the same person be DEAD, with NO feeling in certain areas, and then next time if I get in there just right it's like it sends them over the edge immediately! It's hard to tell....as for your question if it is common for a female LEE to be OFF and ON, and be able to block it out for a bit...YES, very common I'd say.