I have had a similar problem with the Steripen when trying to do a second batch of water right after doing the first. The solution for me was to dry the sensor contacts before doing the second batch. This is quite important!

I have found my Steripen to be quite reliable but to get it to be so, I have had to refer to the directions repeatedly and to read them carefully. Some people with arthritis (like me) can have problems pushing the start button. Others have had problems with the batteries loosing charge if left in the pen. I now use one of the few arthritis-free fingers I still have to push the button and I remove the batteries from the pen between uses.