Try changing whether you are pushing or pulling. Also you may need to circle around to the other side of the stone.
If you are absolutely stuck, you'll need to contact PIXEL support.
On your game screen, choose the Settings wheel in the upper corner. Write down your Player ID and then choose the Support button. Scroll all the way down to the bottom of the page until you see a list of categories. (Do not use the white text box in the top half.) Choose a category and then provide your Player ID, WebGL vs. Mobile, and problem details.
If you are using an iOS device and you do not see the category options on the SUPPORT page, you'll need to email PIXEL Support at: firstname.lastname@example.org