I have never seen a coin mech like this one, but just guessing, I would say the wire needs to bend up into the slot so the coin will push the wire down as it falls through. (See modified image below).
I have no idea why the coin is hanging. Looks like a catch at the bottom of the coin has it. have you tried tying it back?
Thanks all sorted now. For future reference of anyone else who may have this mech (the CM01 Mechanical "Classic" Coin Mech as sold by OzStick):
This shows the coin rolling correctly, the piece that has "CA CALLE" embossed on it needs to be very very slightly below the top level of the coin:

You can adjust this level by unscrewing here and dropping slightly:

Here is the bent microswitch wire as advised by DaOld Man (thank you). Note it is much easier to bend it BEFORE attaching the microswitch:

Simulated action shot!

Coin drops directly onto bent wire, pushing it back enough to trigger the microswitch:

Cheers!
(Please ignore that very distracting "tic tac orange bigbox" in the background of that last picture)