What if my baby does not turn his/her neck in both directions or has consistent one-sided head tilt?

Eye problem? . Head tilt can be a sign of visual or ophthalmological disease. It can also be a developmental problem or neurologic sign of illness. Please have a thorough examination done by your pediatrician. Age and developmental status is an important variable we don't have here to help with an appropriate answer.
Check w/ Ped. The pediatrician should evaluate the infant to rule-out a physical restriction to neck movement ("torticollis"). The pediatrician may order neck x-rays and/or refer the baby for specialty evaluation by a craniofacial or a pediatric orthopedic surgeon. After being certain there ids no physical problem, the infant may be referred for neck physical therapy including a daily home neck therapy program.