Tanaka M40 Trigger issues

[Patrolpointman]

I'm having issues with the trigger on my M40. The rifle cocks fine but after pulling the trigger, the valve knocker doesn't reset- if I push on the valve knocker from within the magazine well, I can see the striker spring within the bolt moving. This must mean the cocking piece is engaging the knocker but for the life of me I can't understand why it's doing this.

Any ideas?