how come some training sessions don't have strength? i'm trying to train my pets but i can't train them for strength

At the training center, you can only train your pets' other stats to 2.5x their level. So if your pet's level is 10, for instance, you can only train the other stats to 25 before you have to train level again. If their strength is already higher than that, then it disappears from the dropdown. You can either train level first or raise their strength with stat boosters