Try the old fans back is one thing you could do. If the fan worked on a
8V battery but not in the power supply that leaves the new fan or power
supply at fault and doing this will point you to the culprit.

It may be a good idea to compare both plugs on the end of both the
fans... in case you are plugging it into a three pin socket and one of
the connections are wrong. Some fans as you may know have three wires
and not two.

davy