Right, the message that says "Something is preventing {girl.Name} from getting pregnant} appears if any of these 4 conditions is true.
* She doesn't love you enough.
* She's already pregnant.
* She already has a kid on the daycare.
* You have not been to the nexus.
So, you have two kids with other girls so 4 can't be.
I'd say wait a week to see if it isn't the 2nd one.
3 most likely not.
So that leaves 1. Check her heart card to see if she's at love level 4.
The exception to the 4 heart rule is Gwen.
If you are stuck on "need to progress with the Organisation" for Jessica. Check a couple of posts down, I'd rather not repeat that too often.