You likely have a poor connection, possibly a rotton wire end at a
terminal somewhere, or a bad switch, etc. The connection can provide
"about" 12 volts with no current being drawn.

We must presume you have used a similar flourescent fixture with
internal voltage convertsion enabling the lamp to operate.

Can you use the feed to light a normal incandescent light?

If not, then I would suspect the power circuit in the flourescent
fixture to be bad. If so, consider an LED style replacement.

Terry K