You've to check the status of Jack's strength. If you are using some of the saves here, then there's a possibility that they've raise Jack's strength to 100 which is why you're getting that response from Sam. If you have you're strength around 35 like me, and all you can do is send Jack to training ground and when you reach there all you've the option is to send her back home. Then do this - send her to training ground, but don't send her back home. After a day, you'll see that her strength is increased by 1+. Do this till you've reached 40, and then you'll be able to get new option for push up training.