If the ball is found to be magnetic,it is worth trying a little heat to upset the magnetic domains within the metal. Not too much heat, the ball housing is brazed to the pick up pipe, so a hot air gun would be a good start.
This will also melt any hard deposits lining the housing.
An alternative thought is that with new, thick oil and a new Super Suck Pump the ball is being drawn to the top of the housing, blocking the pipe, then innocently dropping back down by the time the motor has cooled and the sump plate is off. There needs to be space around the ball for the oil to get by. So a crud lined housing could be the problem.
Carb Cleaner Spray and Nail Varnish Remover/Acetone are a couple of solvents to try....wedging the ball up and leaving it to soak in a small container overnight, for example. Then a good poke around inside to pull out any solids.
But if, as GB says, everything is clean then this becomes more of a mystery of the universe, and changing the pick up pipe an annoying prospect.
Hope it is a bit clogged, if not, maybe worth trying thinner oil, but clutching at straws a bit here.
Swarfy.
Additional. This got me thinking and I have just checked a couple of pick up pipes, and in each case the ball hardly lifts off the seat before being stopped by something in the housing. It comes to rest just 3mm from the flat face of the ball housing, not leaving much space for oil to pass the ball.
I have never had a valve apart, but Roy Bacon's diagram on page 93 in my copy of BSA Twin Restoration would indicate the ball can rise a fair distance. So either the book is wrong, or I have a problem looming.
Anyone with a sump plate off to compare how these valves are set up? Anyone successfully running without the ball?