I live in a second floor condo unit. First floor is the parking garages. A couple of months ago I replaced the old toilet with a low volume, Toto toilet. Since then, every time my next door neighbor flushes her toilet the water in mine goes away and I end up with almost no water in my toilet bowl. I have checked with the neighbors above me (there are 2 more floors above mine) and also with the ones above my next door neighbor and none of them have experienced any problems with their toilets. A plumber says that the problem is that the low volume toilet is not suitable for my building, but these days all new toilets are low volume. My neighbors have low volume toilets. Any clues about what the problem could be and what I could be done to get it fixed?
What's happening is that when they flush, it is creating a vacuum and sucking the water out of your bowl in an attempt to get air. The vent stack is almost certainly obstructed above your toilet's tie-in.
As far as what to do... you are in a condo, there should be owner's association that handles shared infrastructure. If not, find out what is blocking the vent stack and remove it.