Some anecdotal evidence from me:
They called my office. I didn't speak to them; I wasn't available, so they left a message with the person who answered. I don't know if that counted as "getting through to a member" or not. The numbers would suggest that it doesn't, also the fact that they called twice.
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I have forgotten to vote in a few Nominet and building society elections. I have never deliberately not voted, I've just put it off and then when I've remembered it's too late.
I have never forgotten to vote in any 'real' local/general/European/Mayoral election/referendum, nor have I ever deliberately not voted. I have however, since switching to postal votes, often handed in my vote on election day. It helps that it's always a Thursday, and usually the first one in May. And also, if I were to forget, my wife would remind me. I'm not alone in voting at the last minute.
So an unmemorable deadline (and perhaps too much advance warning) is one reason people don't vote, and the second is that people around them aren't voting so they don't remember to do so.
A related issue is that if you're not entirely certain who to vote for, the temptation is to wait a while to come to a decision. Eventually one realises that it's AGM time, and by then it is probably too late and/or the login details may be in a different location.