Do you take it once a day? If so maybe it wears off. Maybe ask your doctor about splitting up the dose throughout the day?
I used to take 30 mg Lexapro, which I was told was a lot. Though I have anxiety, I was taking it more for my depression, since Lexapro is technically an anti-depressant (though anti-depressants can sometimes help anxiety). Lexapro didn't work for me, but its different for everyone.
I'd talk to your doctor, you could also take 25 mg, might be a nice compromise between the 20 and 30. Also, I believe Lexapro is a drug that you have to take for a little while to have it fully build up in your system (I think it takes like 6 weeks to get the full effects). So maybe it is soon to up it? Another thing to ask your doctor. Good luck!