Timing is Everything!
Timing is everything for electronic messaging and sometimes it goes to the back burner.
The Subject, Context, and Kiss are important.
Needless to say, if you need a reply ASAP, you're better off calling. Similarly, be conscientious about the recipient who may be swamped under work or -- even worse -- on vacation? In those instances, follow-ups aren't going to be effective, regardless of how flawlessly crafted.