Insert image sizes disabled

I’m having a bizarre problem when inserting images into a post. The choice to insert “thumbnail”, “medium”, and “large” image sizes is only available when the image is below a certain size. For example, if it’s over about 2700 pixels wide, you can’t resize it at all and must just insert it at full size. 2700 and under appear to resize normally. I have the default WP media settings of thumbnail @ 150, medium @ 300, and large @ 1024. I’ve never seen this behavior before where a WP install wouldn’t just offer the three insert sizes on any image. How do I fix this? I’m running the most current WP version.