I've been guilty of this kind of thing.
The emails I receive can be roughly taxonomized into two categories:
A. Those I can quickly answer while waiting in line at Starbucks, sitting on the bus, waiting for a late thesis proposal to start, etc. These emails usually get a very prompt answer from me.
B. Those that will require me to sit down and dedicate large amounts of time to a reply. These go into a priority queue and I have the best of intentions to try to answer them quickly, as soon as I first answer emails from the funding agencies and school administration; finish that grant proposal or paper due next week; finish that late paper review the editor has been harassing me about; write that recommendation letter for that other student; answer my colleague's email about a discrepancy they've found in one of my old papers; prepare the lecture for tomorrow class, .........
So, I'm standing in line at Starbucks, and get an email from you with a list of questions:
- When is Assignment 3 due?
- I'm having trouble on problem 6. Here are 3 pages of poorly-written work. Can you help me find the problem?
- Are we allowed to work in teams on problem 7?
Now, I could treat the whole email as a Category B, and get back to you with complete answers to all three questions in a few hours, days, weeks, ...
Or I'll fire off a Category A email answering only parts 1 and 3, under the assumption that you'll ask me again about part 2 if it's important to you, or with the intention (perhaps misguided) of remembering to answer the last part sometime later.
What's the fix?
Try to send emails containing only one question per email;
If you must include multiple questions, make sure they're all Category A questions;
If you get back a reply with only a partial answer, email about the missing parts again.