Albrecht and Isabella did have 3 children (Philip, Albrecht and Anna Mauritia), but they all died as infants.
However I tend to agree, that if Isabella Clara Eugenia had married earlier, she might have had a few surviving children, but she probably wouldn't have had a large family.
She had her first kid at 38/39 years and maybe some miscarriages in the six years between the wedding and the birth of her first child.
If she had married around 20 years I think she will have some healthy kids... Sure not the same number of kids of her sister Catalina (who was really fertile and stopped at ten children only for her death. Butterfly that and she will have at least another 5/6 kids or pregnancies).