Issue w/ artwork decentering upon saving/ exporting

Hello. For some reason when I'm exporting my artwork from Illustrator it is not remaining centered. I'm not sure how to fix. I went in and made sure everything was centered, but when I open exported the PNG to post/ use, everything is off center to the lower left corner. Can anyone offer some advice? I'm relatively new to Ai and am only using it on iPad right now. Thanks!