2 yr old still cries at preschool drop off?

I've been taking her to the 1 day a week program for 10 mos...we have no family here and I thought it would be good for her to be away from me a bit. She's been ok for a few months and Wham! She wails when I drop her off! They say it lasts a minute and then she's fine...but why is she still crying at drop off? Breaks my heart. She says she has fun too. I've tried all the tricks...help!