Debuffs stack sleep/immobile. So if you cast it on character and it doesn't miss ofc then it will carry on to the next round if the ninja does it action already. So if you for example sleep and immobile a character turn 1 after the character does it action then round 2 that character will be sleeping and immobile. In turn 3 that character will no longer be sleeping/immobile. Unless ofc you apply sleeping again then round3 the enemy ninja will be sleeping only round 3.