Weird things going on with Canova Royal WC and Geberit cistern

I had a bathroom installed a couple of years ago, by some (it has become apparent for other reasons) mediocre installers. I have always had blocking issues with this loo, but not another one that I had installed at the same time - but put that down to me rather then the loo and that there is quite a long run to the soil stack, but recently something rather interesting has started to happen.
The water level appear lower then normal, and once Ive flushed, the bowl does not empty very effectively, it fills, (throwing up a load of small air bubbles from within the loo) and then slowly empties. Once emptied a regular single air bubble blubs up, about once a second, for a while.
This doesnt seem like a usual blockage, but seems that there is something strange going on with the airfow through the system, and perhaps the levels in the u-bend not being quite right.

Any thoughts?

cheers

Do you have rodding access to the branch runs from the soil stack?
Get yourself some rods and a decent plunger and try a clean out...
The loo; not you

I theory I should have rodding access, as the drawings the installers were working to showed where it needed to be, but if it is there they have buried them behind a plasterboard/plastered wall which was then clad in tongue and groove - done before I had a chance to say 'hold on that was supposed to be an easy to remove timber panel for ease of access to the rodding access'. I have plonged, but not with the sort that you have shown and had thought that I should invest in a better one. Perhaps thats the first actions. Thanks for your suggestion.

The type I posted is much better because you undo the handle as you push the thing into the bowl water, this allows the bellows to fill with water. You then screw the handle back up and push, thus forcing water around the u bend and not air. This is much more effective at flushing the branch runs!