Party Wall Damage – Who is Responsible?

I have a corner unit in KL (near Jln Tun Sambathan area). One of the rooms' party wall has cracks and dampness since early December 2024. It was reported during the defect liability period and fixed. The dampness/water stain became serious few days after the work, and I reported to the defect team again on December 31, 2024 which was the last day of my defect liability period. They came and fixed and it became even more serious few weeks later (serious water leakage). They came and fixed again and the developer is now saying that they will need to conduct an assessment first if they need to be responsible for the continuous dampness on the wall. The wall is between the unit and the external wall. It could be due to external wall leakage based on our consultations with external contractors. How should I go about this situation if developer or the management refuses to rectify the defect?