In my experience, the Firefly 2 has been the most reliable slave I have used but it will depend how you are using them and what you are using to trigger them. If they sometimes work and sometimes don't for the same battery and the same strobes, that can be a cable connection issue so you should end up with a load of photos like this if you check them often enough before caving trips.
Here are a couple of shots done entirely with them - there is more than one Firefly being used in each of these.
If you can describe your gear set-up, that would help to understand what is going on. Photos of your gear with a description would be even better.