I've had the Assassins Blade perk for a while now and every time I sneak up on something and try to attack it with a dagger it does barely any damage. I never once got a written confirmation that a sneak attack succeeded while using a melee but I get them all the time with a bow.
I've tried it with a sword in main hand dagger in second, attacking with the dagger.
I've tried with two daggers in both hands.
I've tried with just one dagger in my main hand.

No matter what I do I never get the "Sneak attack for x15 damage" like I get with the bow, and the damage is always pitiful.

Try holding down the attack button a few seconds longer for a power attack. I've used the backstab to take down a few things that would have been too powerful for me in a fair fight.

Melee sneak attacks are a lot harder to pull off in Skyrim than they were in previous Bethesda games. They won't work if an enemy is searching for you, so you really need to invest in the sneak tree to be able to do it reliably.

The same thing has happened to me once or twice, but I'm almost certain that it's because the enemy heard me right before I stabbed.

I've tried that a couple of times and just make myself look like a fool as I swing really hard to take out 1/20th of their health. I dunno I'm thinking of just starting over with another character. I tried to make an assassin type but if I have no way to assassinate people in one hit like I thought the Assassins blade perk would do then whats the point.

i <3333333333333 the sneak perks...

my guy's level is 22 with archery and sneak perk trees exclusively explored; archery at 70 and sneak at 80, i would normally just sneak and shoot until i finally got the 15x dagger damage and i LOVE it

its actually 30x damage, the base 2x sneak attack plus the 15x sneak perk

you MIGHT not be in range? i don't know, my rule of thumb, however, is to attack when the option to "pick pocket" comes up, and i almost always one shot them, and in that case the assassination animation video always comes up, if it takes more than one shot to kill them, you have to be quick enough to him them again before they have the time to turn around and spot you (the 2nd attack will count as a sneak attack too)

as a side note, i was skeptical about the Silent Rolling sneak perk, and after i got it (so i can grab the last sneak perk) its actually VERY useful. Try it out

Even a regular stab with a dagger gives me the 15x damage, maybe you have to be directly behind them because I've had just a few instances where it just pricks them and they were alerted. Qutayba7 is right though, a power attck seems to always trigger a one shot throat slit killcam for me.

You do in fact have to be more directly behind them, then to the side.