Did you happen to take the switch assembly out and do anything to it at the same time you switched
batteries? I'm asking because sometimes, if you don't put it back together correctly, you can't press the button. For example, the delrin spring base could be slightly out of alignment. If so take it out and try again.
If you didn't and are sure it's the batteries, then you can take a washer (or even all of them) out. But I'm using AW LiFePO4s in mine with all washers in place.