Drag and Drop Interaction - How to make learners finish before submitting

I think that can work but I'd need to see how you have everything set up. Feel free to share your project file if you want me or someone else to have a look to help! 

Here's a not pretty example I mocked up. I had set mine up to have the submit button disabled until the number of drags needed was reached because I didn't want people trying to submit until they are done. I have 4 boxes so I set the number of drags to 4. When I'm working with variables, I like to add a reference box to show what my variable is currently doing. That often helps me with troubleshooting. I remove it when I publish the final file.