Downloaded the latest windows 10 version and the creatures aren't reproducing to keep the population the same as the starting pop. Not entirely sure if this is intentional or not but it results in there only being a single creature after 50 ish generations.
The number of creatures is not decreasing, but they are slowly converging towards the exact same behaviour, so it looks like there are fewer individuals (since they are perfectly overlapping each other). I have seen this happen with certain simulation settings, so try changing the mutation rate, mutation algorithm and recombination algorithm. Are you using "Chunk" as the mutation algorithm by any chance?