I think you need to look at it from their point of view .. some folk (this happens - predictably - more to younger females with pics in their profiles) will log on, and within minutes will have a page or more of messages. Many of these are terrible, many are simply "Hi", and a good number will be from profiles that are clearly not a great match.
I don't think it's reasonable to expect these folk to answer every email they get.
So, instead... When writing your opening message, give it a little thought... How might your message stand out? What might you say that could pique their interest?
You can dramatically improve your reply rate by doing this .. and as for the ones that don't reply or even delete unread .. don't take it personally, just accept that you're not a match and move on!
Good luck in your search!
Used to post as crazyml - might do again if I remember to login to that a/c.