Walk Behind Lawn Mowers

Purchased a Husqvarna HU675AWD, 3/29/14 dirt kept building up in the catch bag making the bag extremely heavy and no longer fit well on the mower sending a lot of dirt and grass clippings blowing back on me. Took it back to Lowes, they replaced it immediately. Second mower is doing the same thing, after only three uses. Is there some way to keep the grass and dust from building up in the side of the bag and to make it a more snug fit? And I'm tired of all the grass and dirt blowing in my eyes for a hour while I mow! I have a quarter acre of hills to mow. I've never had a mower do this. I believe it's just a poor bag design