Our sand bags failed too.

Some clever person here suggested using left over bleach bottles (of which I had MANY) filled with sand. I funnelled the sand into the bottle, then filled it with water to be sure there wouldn't be any air in the bottle. I also put a few drops of bleach in there too, for good measure.

I filled 5 of the 1.42 gallon bottles, each one weighed about 23 pounds. We used cable ties to strap them to the sides/bottom of the stairs.

It still travels a bit, since we don't have it screwed onto the deck, but it's pretty stable.