This happens most likely due to the next reasons:

1. Your iterations number is too low and you don't see those few droplets that were generated.

2. Minimum droplet size is greater than the maximum droplet size.

3. Droplet minimum / maximum size is too low.

4. You have set an empty vertex group as density group 

That happens if the object is too big and the droplets are too small. This add-on is made to accept any object size no matter the time it takes to generate the droplets. That is why really large objects with super small droplet sizes can slow it down a lot. I suggest you slowly input different values and see the limits of your computer.