Setting aside whether the balcony is part of the lot or common property, the issue is whether there is a defect in the common property which is causing the problem. On the face of it, it sounds like there may be.
The Owners Corporation is responsible for remedying any defects in the common property. It doesn’t matter whether it is an improvement to common property, if there is a defect they have to fix it.
I suggest you find a structural engineer who can give you a preliminary opinon on the cause/s of the water penetration, including whether it is defects in the common property that are the cause or part of the cause, and how they can be addressed. Once you have that, you should approach the OC and ask them how and when they are going to fulfil their obligations.