Emails are part of a written conversation, so as you wouldn't leave a face-to-face interaction without saying anything, you should not abandon your conversation with your instructor once they have replied to your first email. This is specially true for requests that involve higher imposition levels.
In addition, don't make your email just one line. Remember to include a greeting, and a complimentary closing plus your signature.
Below you can find a file with an example of a successful interaction of this kind:
Many thanks for your email.
Thank you for the quick response.
Thanks for your help in this matter.
Thank you for your help in solving this problem.
Thanks once again for all your trouble.
Thank you for any help you can give me.
Thank you very much for your assistance.
It is possible that your instructor cannot comply with your request (particularly if it was a high imposition one).
In this case, you can thank them for their reply and show your understanding.
Example: Thank you for your quick response. I understand that...
...you have a very busy schedule.
...the deadline was very tight.
...I should have submitted the work earlier.
Below you can find an example of a succesful interaction of this kind:
University lecturers receive tens of emails every day, so it may be the case that after a few days you have not received a reply.
If you send a follow-up email, remember to keep a formal and nice tone:
Avoid phrasing your message like this: I have not received a reply to the email I sent you three days ago.
Try this instead: I was wondering whether you had had a chance to look at the email I sent you a couple of days ago.