I’ve had this happen several times during a Drupal project. The TinyMCE image uploader button (pictured below) doesn’t show up on accounts that aren’t the “site maintenance account”. Even if the account is an administrator account, the button won’t show up.
But there’s a fix! It’s really simple, but for some reason it eludes me for about 30 minutes each time. So this post is really for my own sake to reference later. If it helps you too, great!
Fix: Go to Configuration > Media > IMCE
Then make sure you add the correct profile to the correct role. If you’ll notice, the “Site maintenance account” already has a profile by default. That’s why when you’re editing it seems as if the uploader button is present, but when you sign on as a different user (even an admin) it disappears.