I sought help with this problem before on here but found no solution. Until now.
Briefly a toilet that had once flushed satisfactorily got to the stage wher e it took a lot of effort to get it flush solids. I gradually changed the e ntire insides of the cistern and the short length of down pipe between cist ern and pan but it made no difference.
Last week SWMBO said she noticed the carpet at the back of the WC was very wet. A quick look showed that the flexible connector from the rear of the pan to the down pipe had failed and had splits in it.
I replaced it and so far the toilet is performing faultlessly, unlike the r ib I cracked while kneeling on the floor and leaning over the pan trying to wrestle the flexible connector on to the rear of the pan.
Yea I forgot to lubricate it with liquid soap.
I can only imagine the splits allowed air to enter and so interfere with th e syphonic action ??