Check your fry very carefully, in my experience fully 50% of an sb to regular spawn will be sb. The catch to that? That 50% will range in length, so perhaps only 25% will be extreme sb as in parrot fish size. As soon as you see suspected sb fry remove them to a separate tank immediately or their longer siblings will out compete them for food. If they don't canniblize them first for being weaker/slower.
The males from these fry may not be fertile but I assume you already know that.
My strain of sb Texas, like 75ish% pure (definitely fertile!) Green Texas...
[video=youtube;pZuS9gjtA2U]http://www.youtube.com/watch?v=pZuS9gjtA2U[/video]
The current generation are from parents that are siblings from an sb to wild/regular type spawn, the very fish in my vid. They were carefully chosen and separated into their own tank for grow out. An odd thing about the current generation...even though 100% of the fry are sb, some are actually quite long...longer than either sb parent! Only a few like that and they seem to all come from only one of the two pairs I allowed to spawn. I have 2-3 hundred nearly 1/2" fry right now; )