That's the horrible thing with these "mood cocktails" you have to find the proper balance and mix in order to get our best performance. And most of it is from trial and error.
Most of my depression meds have rendered themselves in effective at one point or another - I switched up sometimes and then returned. I think it is SO important to communicate with your doc about the specific symptoms so that they can help you in the best way possible. Most recently I was started on a small dosage of prozac in addition to my wellbutrin because I have horrific OCD symptoms. They seem to have gotten worse and I am disturbed by this -- so I have to REMEMBER to call my doc today and not just live with it until things get unmanageable again.
Try not to obsess about whether or not your meds are losing their effectiveness. I know I start to do that. Mood stablizers are different. I have been on various ones and found them helpful - I felt less highs and lows.