No that is not normal. Try closing down the SAI program and reopening it. If that doesn't work try unplugging your tablet and plugging back in and the restarting the program. And if that doesn't work reboot your computer.
OHHHH WAIT! Before you do all that. Check to see if you "Min Size" is set to something low. Personally I keep mine at 0% for the most part. This is probably your issue.
Honestly Sai is a somewhat glitchy program. On minute your sensitivity is working the next it's not. Then you're tablet is not communicating with the program. But still an amazing program none the less.