Dual button flush valve

[Mark1!]

When flushing using small button it empties and fills ok. When flushing with large button for disposal of solids, the fill valve doesn't fill the cistern. The flush valve and fill valves must be ok as they work when flushing away urine using small push button. How can small push button work but largepush button not work when its the same black plastic rod inside dual button mechanism?
Please kindly help

 

[cross thread]

They normally push on separate parts of the valve , have you took lid off ,may need rod adjusted

 

[Mark1!]

My cistern is concealed behind tung & groove. Dual button is therefore located on t&g. Just one plastic 'rod' within dual button device. Dual button rod doesn't directly touch flush valve. The cable connects valve to push button mechanism with single rod. Which rod requires adjustment?
Thanks

 

[blup]

It was only set up for single flush or the rod has failed, if you can’t access it the t&g will have to be removed