I have had a ThinkPad Active Pen (model SD60G957200) ever since I got my Lenovo X1 Carbon Extreme.
The pen, when it works, has worked well. However, the problem has been that whether the pen and PC detected each other seemed very hit and miss.
Today I found the root cause. It was not drivers or dodgy Bluetooth as I had thought, but a weak spring inside the pen. It was not so weak that the battery rattled, but weak enough that the electrical circuit was not being closed reliably on the battery.
The fix was to replace the weak spring with new one out of an old ball point pen. Once this was done the pen became instantly reliable.
Wish I had spotted that sooner.
Updated 11 Nov 2020: I may have spoken too soon, it is back to it’s old behaviour today 🙁
However, I think it could just be the AAAA battery. Seems it is not a good idea to leave a battery in when the pen is not is use given the pen has no power switch.