Putting a bog in a room that used to have one should be a simple task, you'd think. (In a shop premises we took on a couple of years ago, upstairs gutted and largely derelict)
I decided to run a new mains water pipe, shoved the pan into the soil pipe, flushed, plainly a blockage. The soil pipe had been 'capped' with tea towels which I removed (I know, what a ###ing pro) so my concern was that one had traveled.
Much plunging later and we were flushing, game on. Except the flush was going into the ceiling below :( What a mess!
Long story short, there were 2 pipes bossed into the soil stack*, left open when the place had been gutted and upon cutting out this section there was a dishcloth just slightly downstream of the above restricting the downward flow.
So my advice would be to replace a soil stack (or sections of) that has anything of unknown provenance bossed into it.
*Not immediately obvious, they were bossed into the rear.