These bags are fine when they're new. But even if you empty the bag regularly, the fabric tends to get saturated with dust over time. This increases their resistance to air flow and prevents the extraction from working well. I'd advise you to use compressed air to blow the dust out of the bag (from the outside in) when you're emptying it.

If you find yourself doing a lot of sawing, you can build your own dust collector. Here is a really simple one.