I am not certain why they die, as I've read, even when provided with good flow and food, they still seem to die.
Maybe the clams dug into gas pockets in their anaerobic substrate?
Maybe they didn't meet the other necessary water parameters to keep the clams healthy?
Can't say for certain as none of that was mentioned.
Though as far as I can see, if all the water parameters and temperature are right, and you have good flow and provide enough food, they should be able to survive. I am sure some people manage to keep them alive in aquariums, so it shouldn't be impossible.
I haven't fully read up on them as to why they need to burrow, but if they don't seem to be able to burrow in your substrate, at least try pouring a sand area in a place of good flow so the clam has somewhere it can burrow (preferably with good flow so they can still filter feed in that area).