Eraser tool won't work on image placed in existing photoshop file

Hello,

You could say I'm an intermediate-plus on Photoshop. I have an issue that's bugged me for awhile, and can't find a specific answer.

Issue: When several images are placed in a single PS file (separate layers of course), the eraser tool requires me to rasterize the image before it can work. But when I drag and drop the original image into PS it works perfectly. Easy, and I know how to do that. I'm simply knocking out the background.

• Ideally, I'd like to place these images into a single PS file on separate layers, then remove the backgrounds. 

• That file is set up to specific pixel dimensions I need for this purpose; i.e., all images need to be proportioned within this width X height.

• Trying to avoid having to drag and drop each into a separate file to do this – one PS file vs. having to work with several drag and drops.

I hope this is clear. Any advice would be very much appreciated (even if I feel stupid, LOL)!

Thanks in advance!