When using
/summon FallingSand ~ ~4 ~3 {Block:cactus,Time:1,HurtEntities:1}To try and make a 3 tall cactus, It only makes a 1 tall one, which will kill the falling sand entity before it places a block, or cannot place a block for some other reason.
Copy the above command and run it 3 times to reproduce.
That is because when sand falls onto a cactus, alike to many entities, it will be destroyed, and so it cannot place the block, leaving only the first falling sand in place, with the others getting destroyed and becoming items, which most likely also got destroyed by landing on the cactus.