WC not flushing away toilet paper even with a bucket of water

[Bertie]

Got a customer who's toilet will not flush away waste on the first flush, it can take several attempts before its all gone.
I have tried flushing it from the cistern via the syphon and also with a large bucket of water directly down the pan, but either way it does the same thing. The toilet paper seems to hang around at the bottom of the pan.
I have checked for any obstructions within the pan, vent and any soil pipe-work.
I have checked the vent pipe for any blockages and everything is very clean and clear.
The customer said something which made me think, she uses a lot of bleach down the toilet and somebody had told her that this over time removes the enamel. Is this true?
My next step would be to replace the toilet completely.

Any suggestions

 

[Phil]

Have you checked the pan outlet by removing toilet?

 

[Bertie]

Have you checked the pan outlet by removing toilet?

I have checked pan and it's clear.
Soil pipe is clear too from toilet all the way to the branch on the vertical part of the stack.