Photoshop :: Recorded

I'm trying to set up Photoshop to create a bunch of thumbnail images. I've got actions to select various preset sizes with the marquee tool, and then one action that copies, creates a new document using the clipboard preset, pastes, and then resizes the image to 150 x 150 px.

The problem is that the resizing seems to have a mind of its own. Sometimes in the new image I get the selected area with a big blank space surrounding it, and sometimes most of it gets pasted outside the canvas so only a small part is visible.