Hi - I see you replied to the email sent by the forum account alerting you to the response - in the future, you'll want to post replies in the thread.
You said in email:
I tried adding a validation rule, which applied only when the drop down boxes put the required fields in view.
So, the validation rule for each field would have two conditions - one being that the field is blank, the other being that the field bound to the dropdown control was a certain value. Were those the conditions you used? If you like, you can attach your form to a reply forum post (under the Options tab) and let me know what field you have your validation rule on, I can double check it for you.