Many defensive abilities (that increase counter chance) also add their own counter-attacks to your counter pools when used, taking priority over passive counters like the one from the Dustbag item.
Unfortunately, when you have multiple counters available, the priority is automatically assigned to the highest counter in the list (which for Counter-Sand the 2nd lowest, only above Counter-Swipe) so the Dustbag counter is less likely to proc than pretty much any other counter.
To get it more often, try unequipping anything else that adds a counter-attack, which may include equippable passive skills as well as items.