Mark, I can tell you EXACTLY what happened - the pores in your HEPA Cloth bag became clogged and plugged up with fine dust. That's why you had no suction or airflow, yet felt tons of air coming out the HEPA Filter.
For anyone reading this, here's a tip - every time you finish vacuuming, take the bag cover off, give the bag a good vigorous shake, and put the bag cover back on. If you're cleaning a really large area, halfway through, turn off the vacuum, shake the bag, put the bag cover back on and finish vacuuming. If you do this, the vacuum bag will allow the cleaner to retain it's suction and airflow MUCH LONGER, and you will be able to fill the bag at least 3/4 full before it ultimately needs changing.
Devin - "Keeps your airflow for longer, with less dust escaping into your vacuum". Sorry, I have found that NOT to be the case! Yes, they do eliminate dusty bag chambers. And I ALWAYS suggest to anyone looking to replace a cleaner because someone in their home has allergies, to try putting a HEPA Cloth bag in first and see if that makes a difference for this very reason - with a HEPA Cloth bag in a vacuum, the only dust left in the air is the carbon dust from the motor pretty much. As for allowing the vacuum to keep it's airflow and suction longer than a paper microlined bag? Nope, they still allow a vacuum to lose it's suction just as quickly as with paper microlined bags.
The ONLY HEPA Cloth bag I have found that TRULY allows a cleaner to retain it's suction and airflow as it fills is the Miele GN, FJM, U and KK HEPA Cloth bags. You can fill those bags till 3/4 full without having to do that trick I mentioned above. And when you look at the bag, it's quite obvious why - LOTS more filter layers! 9 layers of HEPA Cloth material, versus 3 or 5 layers for other branded or generic HEPA Cloth bags. It's the reason why I will adapt any cleaner I own, when possible, to use these bags. And in fact, the ONLY new cleaners that will come into my collection, with the exception of Aerus Electrolux cleaners, MUST be able to use a Miele bag.
As far as I'm concerned, the Miele Airclean bags truly are revolutionary, and when they were introduced in 2010, they completely changed the playing field amongst their competition. I can only imagine how much better all vacuums will perform when the patent on them expires and other manufacturers start producing 9 layer bags of their own.