Hey Meghan, "Find & Replace" can be tricky because the text entered (or pasted) in the bulk edit "Find" text area needs to be an exact match (including spaces and line breaks) to what's in the existing descriptions.
The good news is that you'll know if there is or is not a match before needing to click "Apply" and "Sync Updates", because the existing text will be highlighted (in blue) as it is entered into the "Find" field, and then the new text will be shown (in green) as it's typed.
My first suggestion would be to type the text you'd like to find into the "Find" field rather than pasting it. This will increase the likelihood of the two matching. However, if you still encounter any issues, please send a note through the our chat tool by clicking the blue circle in the bottom right hand corner and I'll investigate further.
Thanks!