First off you have made great progress getting from 28%BF to 21%BF, dont be so hard on yourself, well done. 21%BF is a healthy fat level for a female.

Do you feel the Maintenance plan is not allowing you to reach your current goals?

You mention when you up the carbs you put on weight, do you reduce your fat intake? do you make them complex carbs or simple carbs?

Have you tried following the BFL way of eating? if so what sort of results did you get with that?

Don't get too hung up on scale weight, you can lose fat but still put on weight as muscle is more dense than fat, so a small amount of muscles weighs more than the same amount of fat.

Mike