first of all, its not waterproofing that warns out, but the DWR(durable(?) water repellent) coating. The "waterproofing" is on the inside of the coat and is a fabrics that doesnt let the moisture get to the next layer, however lets the jacket get soaking wet. The DWR is on the outside and makes all the water bounce off the fabrics...
You get wet not because the waterproofing is gone, but because the DWR is gone... When there is no DWR left on the jacket, it gets soaking wet(however you are still dry on the inside) and therefore not breatheble. The moisture from your body is condesating on the inside of the jacket and thats why you feel and get wet.
To restore the DWR coating, you should buy a special tech wash or spray works great aswell. I think you can find it in most of the skiing stores. Or any of the sailing/camping stores should have it too.