The calculation of 500,000 divided by 37854ml is 13.2 so probably pretty close.I've heard this is due to either environmental stressors on the clowns, the eggs not being fertile, or the eggs being damaged/unhealthy - if this is happening while the clowns are still spawning regularly, I'd guess it's not environmental stress, so I'd look at the diet the clowns are eating to see if the clowns are getting adequate protein, fat, etc. for the eggs/larvae to develop properly. No guarantees this is the issue though.
Optimal rotifer stocking from what I've seen is ~10 rotifers per milliliter; 500,000 rotifers in 10 gallons would be ~132 rotifers per milliliter, so (assuming you have that many rotifers in the 3.5L) that would be too much.