Ive got a Casio keyboard that needs 9 volts dc current.

Ive got about 10 voltage transformers.... many of which you can set
to 3 v 6 v 9 v 12 v

strange that none of these seem to get the Casio keyboard going !

If you've randomly connected it to incorrect supplies then you might have killed it? - you also need to be aware of the correct polarity - incorrect polarity may well blow it instantly.

There's also different plug sizes, so they might not be making a proper connection.

YES Im aware of the polarity , they often have a split point in the wire where you can unplug and turn it around.

You might have a point with the plug size....

but having gone thru all those adaptors, you d think one would have kicked in !

could it be that they are supplying the right volt.... but not enough amp for
the keyboard which is quite big

its a casio CT 680

AC adaptor AD 5

Possibly, but full power probably isn't needed other than for running it hard and loud.