If you aren't getting more than 40 hatch from a spawn of 200, the eggs aren't being fertilized. I use enough Methylene blue that it is difficult to see the eggs, so that is not likely your problem.
Condition of the parents could be a factor (this is important), as could the substrate they are laying on. As an example a friend of mine was getting a poor hatch rate. He watched the male follow the female and deposit the sperm far from the eggs she just laid. His solution was to narrow the slate he was using and that helped.
A high flow rate in the tank could be a factor as well.
Join a fish club. Don't have one in your area? Start one.