I live in one of three units on top of a commercial space. The building is about twelve years old. We have two toilets both of which have to be flushed twice if there are any solids. Our neighbours also have a similar problem. One interesting thing we have noticed is that flushing one toilet cause the water in the other to bob up and down slightly. I was wondering whether there is a vent problem, not enough slope on the waste line or what. All three units are vented separately. What would be our best way of determining what the problem is? Thanks in advance for any assistance in this matter.