It sounds to me like you just need to space your hits out more. There is a lot more stun than you think between the hits and you may be negative edging the bugs too close together. It's been said before that 214a and 236b and pretty interchangable, and I feel that 214a should only be used when you have to make up distance or all 3 hits of the 6c loop wont hit.
With this being said, you should watch (starts @3:05): http://www.youtube.com/watch?v=FJY2apOFmDw (thanks to Senkai pg 1, post1). While this combo starts with 6a, 5d rather than 2a, 5b, 5d, the mixup before it pushes the Carl far enough that he needs to use 214a to gain some distance before 6c looping. But you can see how he spaces all of his hits ( and bug hits) pretty evenly and keeps the player stunned in a combo as long as possible, allowing as much time as possible for 6C to come out.
Just give it some more practice, and I'm sure you'll get it.