1) If they have Ink Armor, their Ink Armor will absorb the roll and bounce you off before wearing off. Ink Armor is a new special in Splatoon 2. You can tell when an inkling has Ink Armor by their glowing eyes and the smoky ink they're covered in.
2) Are you sure you're using the splat roller? Carbon roller can roll but it doesn't do 100 damage while rolling.
3) Latency issues between you and the target may cause the game to not register certain attacks correctly 

 this is something that happens sometimes in Splatoon 1 as well, and will hopefully be very rare.
Ink resistance is in the game but does not give you the ability to take roller rolls without dying lol.
Aerospray MG has the Curling Bomb Launcher, which lets the user throw Curling Bombs for free for a limited amount of time.