One of my children always gets fevers after a vaccine and the others don't. why do only certain children tend to get them? is their immune system weak

May be a strong immu: Can't say - you may have a child with a strong immune system. You can have your pediatrician have the child seen by an infectious disease pediatric specialist.

Normal: Every child's immune system is slightly different. Some kids get fever, others don't. There is not reason to see an infectious disease specialist or an immunologist.

Fever and muscle soreness are typical symptoms after a vaccination, but the symptoms after receiving a vaccination do not reflect necessarily the antibody responses. The immune response is most likely normal for all of your children.