My Green Sport Strap did this when it turned up (the classic leather buckle pops in easily).
On mine it seemed there was a little resistance to the pin moving, possibly caused by the steel pin rubbing against elastomer rather than steel against steel with the leather band.
I found that if I popped the strap into position, then pushed the eject buttons a couple of times it locked into place nicely and over a period of time I could pop the strap on and not have to do this.
I do check that I have a good positive lock when I'm using this band every time now.
I'm just wondering if this is actually because the strap is such a tight fit in the steel watch and compressing the elastomer might start pinching at the pin.