Does she have a saddle or are the eggs already in her swimmerettes? If they are already in the swimmerettes, that means that the eggs have been fertilized. Shrimp develop a saddle first and then molt. After they molt, they release a toxin out into the water. Males go crazy when they detect it and can be seen swimming around frantically. After they mate, the female transfers the eggs to the swimmerettes where they will eventually hatch. If you do not detect any baby shrimp, it could be one of many reason. Some common causes are they hatch into a larvae form witch requires salt, they are getting eaten by fish, or the filter is killing them. It can also be a combination of these.