There are certain people I do the read receipt with. I didn't know anyone actually used the priority flags though.
I use the priority flags. I think I get a good response back because their emails go red. Make sure you set the flag for the recipient and not yourself and for a time that is during working hours. That's where some people mess up when "flagging".
I don't bother with "read" receipts because they annoy me and I click ignore on the pop up when it asks if I want to send back the receipt. So I know others have to do the same.
Thing is, people get busy. Many people wear multiple hats at work. They are pulled into last minute meetings, they have "emergencies" that pop up, crap happens. When prioritizing, stuff gets juggled around. Sometimes emails get lost in the process of juggling. Sometimes they just aren't at their desk to return those phone calls. I know this from first hand experience, as I have 4.5 jobs that I'm covering in addition to the one I got hired for. I had an entire week where I was unable to get my voice mails and I wasn't at my desk. By the time I checked messages, I didn't return the majority of the calls because I figured they were OBE.
So I always try to remember, that document I need reviewed "asap" may be extremely important to me, but it may not be as important to someone else. Put yourself in their shoes.
Keep your emails short. If I see a long email, I may skim it and not catch everything. Same with voice mails. I may delete it before you finish rambling on, because nobody's got time for that.
Also instead of saying "comments due by x" say "I need your comments by x. If you don't have comments, let me know." It helps to at least cross them off.