I don't know what happens. Agents are, in my experience, hard working people in love with books - they have to be, or they couldn't do the job. They're normal (although I'm sure I once spotted a dorsal fin), so why does it seem so hard to write a normal, straightforward letter introducing yourself and your book in normal, straightforward language? It must be the weight of the thing, summing up possibly years of hard work and hope in a couple of paragraphs. Well, five...
1. Why you're writing to them.
2. Brief summary of your book.
3. Market position of the book.
4. About yourself.
5. Thank you for your time etc (I call this the 'I am not a loony' paragraph).
Pop it all onto one page, and there you are! It's not difficult. Except it is. My first agent letter is the one thing I've never shown to anyone else, so ghastly and needy it is, I might as well have disembowelled myself and sent the contents by Parcel Post. My only excuse is alien abduction. The proof is out there.