Drtay
am just thinking aloud here - would it be possible the developer did a short cut and instead of providing one pipe for water and the other for waste, they
only installed one instead - so the units on the ground floor will get a backflow because the pipe cant handle the outflow of water and waste ? so the ground floor
units will be affected the most as compared to the higher units?

Also if its still underwarranty - suggest you write officially to the lawyer who is holding the remaining 5% (or whatever the amount is) outlining the problem and unless and until the defect is addressed satisfactority the lawyer is obligated under the law to hold back this money - this should be enough to force the developer to deal with the problem. after you have done it then only inform the developer and i reckon that they will be more cooperative after that..get a lawyer to draft the proper notice to the developer so that it will hold up in court in case you need to litigate. most importantly you must do this before the warranty period lapses and asap..
however please double check what i have just mentioned to ensure that this is indeed provided under the law as am no lawyer nor expert..