Speedo drive only spinning when front wheel is rotated backwards

[BAYSA]

Okay bike didn't come with a Speedo or tacho when I bought it, so IpI up an analogue speedo which I've tested and works fine, bought a brand new speedo wire aswell. Did the test of putting a matchstick in the front wheel, lifted the front and span it forward ... Nothing. Out of curiosity I span the wheel backwards and whaddya know the matchstick began to spin.
Now I know what you're thinking that it's maybe on the wrong side? Well the other side doesn't the little nub which holds the Speedo drive in place.
Any ideas?