Sometimes something weird happens where the cursor has an icon next to it, and I can’t clone frames anymore (by dragging them, I can still clone them with the hotkey Ctrl+’). I assume I’m accidently hitting a hotkey that does this? I don’t know.
It sounds like you could have accidentally hit T. This is for Toggling between Add Clones mode and Slide Mode. Default is Add Clones mode, were you are able to drag to add or remove clones. Slide mode is for sliding the original drawings up/down through the “sea of clones” – adjusting timing of your key frames.