If your unit is still under the defect liability period, you should lodge this to the developer. Otherwise, you have to chase the management office or knock the door of your neighbour upstair.
I think u have to show copies of letters that u sent to JMB/MC (with picture showing the leakage or effect of the leakage to your unit) and also issue an official letter to the COB and get them to chop & sign to acknowledge receipt of your letter. Follow up from there.
If no action from COB after a few follow-up ..... MAY BE u can "broadcast" it to the local newspaper and make a complain to HBA