Anyone find Zoloft dosage loses effectiveness?

I've taken Zoloft probably for about 2 years. 50mg. It made me feel better, or I should say just very normal, not full of tension & anxiety & more patient with my kids. When I didn't refill in time I went without it, felt fine, so kept going without. I've now noticed that I'm better when on it, however, has anyone ever found that if you go off for a while, and then back on, the effectiveness is not the same? I'm thinking that I probably need to have my mg. increased. I know that I have to speak to my doctor, I just wanted a "fellow zoloft user's" opinion. I also don't want the dr. to admonish me for going off and on, on my own!