One of the biggest factors in survival success is water chemistry... If you water is soft with few dissolved solids they most likely will not take hold not matter what. If it is usually turbid(muddy) again, they aren't going to do well.
Rocky areas with 1/4" to 1/2" crevices and vegetation are both great structure and escape cover for grass shrimp.
If you have a pond with the right chemistry, which most pond owners on here will along with the above mentioned habitat, you should strongly consider stocking some.
