Many children have difficulties with the development of sucking.
As you have already mentioned it is important that this be properly looked into as there could be several explanations for your baby having this difficulty. Depending on the nature of the problem there is a variety of techniques to develop sucking. Referral to a Speech Therapist could help you to work out what specific difficulties your child is having. Many medical professionals are unaware of the input that a Speech Pathologist can have with early feeding difficulties.