SB:SS can miss, I'm talking about the normal unglyphed Soul Swap when you swap dots from one target to another. This one can't and will never miss. And the 3 dots will always be applied. Just go and try it.
Just checked again, Soul Swap can miss when you first cast it (with the Glyph you get the 30s cooldown but don't get the Soul Swap buff ), but Exhale cannot (and you won't lose any dot). That or I'm the luckiest man on earth to never lose any dot over 200+ casts with no +hit.
I've been having the same issue, it happens every once in a while, usually during dailies. SB:SS applies Agony and Corruption but no UA. I am hit capped however a portion of my Hit is from Expertise, I can't imagine that would effect anything but just thought I throw it in there.
It has been a known issue since mid cataclysm that if you soulswap onto a target that is going beyond max range, like if you're on your flying mount flying past and stuffs, then all, some or none of the dots can be applied, depending on latency and distance.
There's missing, shown in combat log and stuff, and then there's <nothing>. If you encounter weird mechanics like this, try to repeat it and check the combat log, if the first one got flooded.