Why don't they taste the same once the packets been open a day or so and pegged or they've been put in a tin? Don't seem to have the same crunch and go a bit sickly. Might just be me.
To be honest they don't a few days in my house. These little moreish buggers are not lasting long at the moment, especially when they are offer!!
You probably keep them somewhere humid. Kitchen? Try leaving a small pot of salt nearby to see if it goes sticky or turns into a liquid
One of my favourite treats is to get two choccy hobnobs and put them together with a layer of peanut butter, like this.