Not to mention how many times the barrier breaks when he's on Earth, not just the shuriken!
Like when he went out into the hurricane to fix the generator, and has to haul himself back to safety, slipping down rocks until he hits flat ground, all in hundred-mile winds, with a broken shield. And he doesn't even get it fixed right away!
And when he does get back, he collapses in the stasis room after kicking out the scientists, I'd assume falls asleep in there, and then gets up to have a chat with Honeycutt...
all while the glass is still broken! Heck, Shredder deals some blows to the shield, even in the Big Armor, and it's still mostly brushed aside until it's actually shattered completely in the last fight with the turtles.
Though, Krang seemed to have lost quite a bit of his basic logic over the years after the other Utroms went into stasis. You shouldn't have to be a intergalactic warlord to:
1. Make sure your shield against air you can't breathe is absolutely completely unbreakable
2. Not torture a Toothy Mutant Muscle Tank because you had a bad day. "Never tickle a sleeping dragon"
He got pretty darn careless in his last decade...