replaceitem not working twice

I set up a fill command clock, as set up this command

replaceitem entity @a[score_glass_min=1] slot.armor.head glass

I set my 'glass' score to 1 and found a glass block on my head. I took it off and it was not replaced. I got rid of my glass and it still didn't. I even reset my score to 0, then back to 1; still nothing. Finally, I reset my clock, and my score, then redid the whole thing to the same effect: nothing.