It is not hard provided that you made the wheel at least 4" tall (ideally 7" ish), and you press under whey to really fuse the curds. And you do a proper rest in the cold room before moving for eye formation. I wrote a long reply on eye formation and late blowing defect in a thread here on alpines; take a look, it covers a lot of ground for the nuances (high drain pH, moderate brine pH (~5.2), etc). You're not after keeping the gas in, you're after forming few nucleation sites and controlling temp during secondary ripening that produces the eyes. The bag wouldn't help much if your rind isn't conditioned and if your curd wasn't fused properly.