My upstairs toilet swirls but won't flush, though the bowl usually empties. When the upstairs toilet is flushed the downstairs toilet bubbles up, sometimes to the point where water splashes out of the downstairs toilet. The downstairs toilet flushes perfectly. We recently remolded and the downstairs toilet went upstairs and the upstairs toilet went downstairs. Therefore there is not a problem with the toilets. The same thing was occurring prior to the remodeling. The drain for the downstairs kitchen sink goes directly to the septic line. This drain line became disconnected. When it was disconnected the upstairs toilet flushed much better. I then put a vent in the kitchen sink drain line and reconnected it to the septic line. After reconnecting with the inline vent the toilet went back to just swirling. Any suggestions??