Ours is the other key, which also did this, but in the opposite direction.
It locked away but wouldn't lock in the use position.
Inside the case is a small pin, which drops into a tiny groove.
One groove to lock the blade away, another groove to lock the blade in the use position.
The pin is metal, but the groove is only plastic, so over time wears out.
The edge of the groove becomes tapered, so although the pin drops in to the groove, the taper allows it to spring straight back out again.
You can refashion the edge of the groove with a hot craft knife.
Or purchase a replacement fob from such as fleabay.
Eitherway, it does involve taking the key apart i'm afraid, but it's not too difficult.