Hi Nigel, can I please confirm that you are referring to the Amtech 3-in-1 staple gun (stock code: B3750)?

If so, then the Amtech 6mm staples (stock code: B3771) are the correct staples for this staple gun and should not cause any problems. 

Were the 6mm staples you purchased genuine Amtech branded ones. We can not guarantee that other branded staples will work with the Amtech staple gun. I trust this helps.