Lack of maelstrom in my toilet ;)

I know that this was answered by some people sometime ago but my situation is slightly different and would like some more input or suggestions. I recently had some plumbing work done - adding a second bathroom to the house. The toilet is not installed yet but the shower is - sort of (not fully functioning yet). Since the plumbing work has been done my upstairs toilet clogs almost everytime someone uses the toilet. I have checked the vent on the roof and that is clear and the toilet has been snaked for (U)nidentified (F)loating (O)bjects. I have also noticed that the bathroom sink is slow to drain and bubbles back nearer the end of the drain cycle which led me to think that it was an air/vacuum issue, hence the checking of the vents.

Now my thoughts turn to the lack of a toilet downstairs and I was thinking that because of a lack of back / head pressure from the toilet that is not installed, that this could be the cause. Has anyone else experienced this issue before or am I heading down the drain here? I have tried to attack this issue from every angle and now I’m at a loss.

Any suggestions / ideas would be appreciated

Definition: mael·strom [meyl-struh-m] a large, powerful, or violent whirlpool.
For those that don’t want to look it up