They are supposed to open and seal a hole in the combustion chamber. If they aren't doing that it's because the valve is either burnt out, the seat is eroded, or something between the camshaft, the pushrods, the rockers and the valve is broken.

There's nothing that can be applied with a spoon that'll fix that.

The method Bertie describes to hold the throttle wide open when cranking (not pumping the throttle, just holding it open) I understand is written in the handbook for the car.

I'd suggest it is either the coil or the valve clearances that are likely to be causing the poor hot starting.

Yup, but a well cared for engine and carb shouldn't let you down.

aircooled boxers are all a pain to start when hot

Drivers manual says
"Fully depress the accelerator pedal and keep it there throught the starting process, engage starter, release throttle when engine fires"( or words to that effect)

always worked for me ( but tappets and coil main suspects if the car still not starting properly when correct technique used)
Sean