I've taken Zoloft off and on since 1994 (probably 5 years off, total, with sporatic & intermitent rounds of it in between). I have heard that after a while, some meds can kind of "wear off" or be a bit less effective. I do think if I faced the stuff I went through in 1994 with the 100 mg. I usually take, it wouldn't cut it. For most things now 100 is fine, but not always.
Some women find that the monthly cycle can increase the need for a higher dose, for about a week. Not everyone does increase their dose, but I know women who do, and they find it can be helpful.
More or different stress can "tax" your dose as well. The published range of dose is up to 200 mg., but I know a few moms who have been up to 400 mg., rx'd by a psychiatrist.
What's been going on?