Rimeheart has a 10% change to proc on frozen enemies.
Frostburn have a 50% chance to freeze enemies; I see you added Azurewrath and Ice Blades, so you're at a higher proc chance (I'm not sure if they stack multiplicatively or additively, but it doesn't matter). Also note that for Ice Blades the enemy needs to be chilled first. The problem is that all those are CC effects, and as such they trigger the diminishing returns on CC and make the mob virtually CC immune after a few hits. So adding just more and more won't help you past a certain point.
Those procs have to be multiplied with the proc chance of the skill/rune you chose - in your case Ice Blades (31% proc coefficient). I'd strongly recommend to switch to Magic Missile, as most runes have a 100% proc coefficient and you already have the Mirrorball which helps tremendously as well.
So let's picture the fight: first hit either freezes (yay!) or doesn't. If it doesn't, not only will Rimeheart have no chance to proc, it's also getting increasingly difficult to freeze due to CC resistance. If it does, you still will only see a proc in approx. 1 of 32 hits (10% Rimeheart proc chance, 31% Ice Blades proc coefficient). And that certainly feels like "never".
Also another word about Rimeheart. I love this sword and would love for it to be useful. I've tried to create builds for it for a long time. But Rimeheart is incredibly underpowered and useless - it was right from the start. 10000% damage with a 10% chance and a requirement for the target to be frozen means that even if all mobs were frozen, it would only be 1000% weapon damage (which is nothing), and due to the recent CC changes it's impossible to keep mobs frozen for more than a few split seconds in any longer fight. Add to that the many low proc coefficients... effectively, even useless weapons like Stalgard's Decimator or Fulminator have a higher "real" damage output than any Rimeheart build.