No blockage but WC won't flush!

[stephend]

I very much hope someone can explain what's happening here? New'ish build 2/3 years old on three floors. 2nd floor On-suite WC/shower/sinks all flush away OK. 1st floor bathroom bath/WC/sink all flush away OK.
Ground floor under stairs WC, the bowl just fills up although the sink empties OK. The WC connector 90 deg. to the left into a floor to ceiling box which houses the soil pipe from the two floors above. Now here's the mystery.... Pulled out the toilet to disconnect the WC connector, no blockage in the short horizontal run to the soil stack and no blockage in the S bend of the WC. Reconnected everything back together, flushed the toilet and the water shot away as expected, second try just to make sure and water vanished - lovely, except there's water on the floor. Flushed the toilet again and there's water leaking from the WC connector and the horizontal soil pipe. Thinking about it there was no rubber sealing ring around the WC connector so everything was taken apart again the rubber sealing ring was extracted from the horizontal pipe and refitted to the WC connector. Reconnect everything back together and flushed the toilet...... water remains in the bowl!!! Everything taken apart and checked... All OK, reassemble with a flexible unblocking wire going through the WC, the WC connector, horizontal pipe and down the vertical soil stack flushed the toilet ... and yes the water remains in the bowl, the flexible wire removes easily. What IS happening? Is there a vacuum somewhere, after all the other two toilets using the soil stack all work! Any explanation would be gratefully received.

 

[Dsp]

Is the soil vented outside or has it got an Air Admittance Valve??

 

[plumben]

Sounds like it might need a AAV fitting if it flushes fine without the seal letting air in,

Is there a AAV in the loft/top floor bathroom or is it vented outside?

 

[snowhead]

You have a blockage in the soil pipe just below ground level.

Not completely blocked, but enough so it wont let more than 2 flushes of the lower WC through immediately.
After a while it will clear.

When you drop water from the upper floors, it's backing up in the waste pipe but you won't notice it and it's travelling faster so will force it's way through the blockage.

Stand in front of the downstairs WC and get someone to flush the upstairs a couple of times, you'll see the water in the downstairs WC lifting slightly.

 

[stephend]

Many thanks for the prompt replies it's much appreciated. Dsp & Plumbem, I had thought about that one, the soil stack goes through the roof and is open vented. Snowhead, l'll give that a try.
Once again many thanks for all your inputs.