Are they perhaps flagged as pregnant already? You only get the animation for the impregnation once. You can check the heart icon below the face image in the CG room to see if they're already pregnant.
There is no order or other requirement. You have sex with a girl, you have a chance to get her pregnant.
I double checked and she is not flagged as preg. In my save, Shanna is preg. but Naomi isn't flagged as preg. (never was nor I interrupted the preg.) And I might be wrong, but if she was flagged (even if it wasn't showing), in the "time skip preview" it would appear in the "preg" act, the next CG scene. Thanks for all the help.