"Snap to elements" has stopped working

Not sure what the feature is called, but in my indesign file, I used to be able to snap elements in the file to other elements, like aligning the left edges of two images, or scaling an image to hit the edge of an object. I can still snap to smart guides, but snapping to other objects in the page was a really useful feature and I can't find an answer anywhere on how to toggle it on or off.