What could possibly be wrong with my gun stapler?

I have been using my gun stapler with new gun staples that I bought. It has run out and I have now tried to refill it with the same staples. However the staple gun seems to have shrunk or the staples have grown. How can this be? Have I moved something on the staple gun to make it smaller? The staples are 6mm,but the staple gun takes 4-8mm. It loaded the first time without difficulty. I can't figure out what is going on. Can anyone shed some light?