I have the same problem as RisaCooper, sometimes mobs do not count into my exp chain.
Like..when I'm fighting mobs which are rank 29 on my PUG rank 27, those monsters should trigger the chain.
Sometimes I have to kill up to 3 of them until the chain starts!
Or even if it started with the first mob down, if I go to the next one and defeat it during the time left on the timer, it's not recognized by the game and the timer continues to count down..
This is especially maddening when I'm able to string together a 3-chain and manage to kill a 4th mob in time, but the game ignores it and goes like "Oooh, time is up, you didn't manage to kill 4 monsters in a row." although I actually did.

Well, but usually 3 is the possible maximum for me anyway
After that I have to rest. I'd love it if there were adjustments to how hard it is to get an exp chain as a solo player.