Hi @Michelle_Tran, welcome to the forum and sorry for the trouble here!
You can export up to 2,000 tasks when exporting to CSV. For this reason, if your search results or project have more than that amount of tasks, you may not see the all tasks in your CSV file.
Please note your CSV file will include complete and incomplete tasks. You can use the advanced search feature to create reports and split your project or search with specific parameters. This will allow you to export all your tasks.
I hope this helps! Let me know if you have any questions!