Does the valve leak around stem when fully 'closed' or fully 'open'?
The shut0ff valves are 'double seat' valves.......valve disk is double faced so that it closes against a seat in full close & full open positions.
Fully open valve tight against backseat to stop any stem leaking, unless backseat/valve disk face are buggered up.
With it fully open..tighten valve stem packing gland nut slightly (not too much or operating valve will become very difficult).
Use up propane in tank and then have valve replaced....if it still leaks when opening/closing