I have the same problem. For a brief time I was 2nd Lt in vanilla, then dropped back down to MS and then to sgt. I've never been able to stay for more than one game.
Basically, MS and Lt are divided by multitasking ability and critical decision making skills. You may know your civ really well, have excellent micro in battle, etc. but if you can't juggle all the major aspects of a game and process and respond to situations quickly, you won't get very far in RTS. Using hotkeys and time-saving measures helps, too. But if you don't use the time you've saved wisely, it's like you never had it at all. That's where a lot of sgts (including me) run into trouble. I know how to herd, use waypoints, hotkeys for buildings and units, counter system, eco upgrades etc. but I suck at multitasking (as napoleon says) and tend to get fatigued after about 10-15 minutes of microing.
It's so bad it's influenced what civs I play so I can rush and be done with it, rather than have to remember to build houses, manage eco, etc. If my rush fails, my civ is usually in such disarray it is impossible to recover.
Crunkatog on ESO
Bart331 balance suggestion: aztec: remove civ
Voltiguer: Ender, Sioux in 1.04 will be a top civ, no matter how many layers of Sioux goggles you put on
schildpad on Elephants: ...their mansabdar unit sucks so hard it looks like a black hole
Crunkatog on Steam.