Modafinil not working

Has anyone else noticed that Modafinil will sometimes just... not work at all? I take 200 mgs in the morning and it usually helps and clears symptoms almost completely, but every once in a while it will do absolutely nothing.

For example, this week I have taken my usual dose in the morning and have not had symptoms, but this morning I took it and was sleeping within like 30 minutes. So frustrating, maybe its just not the most reliable option for me.