How odd? I was just about to ask this too.
I sent a pdf from Calibre to my kindle and it arrived almost instantly. I noticed there was something missing so I amended and resent, 30 minutes later it still has not arrived.
I will send again, the other day I sent 8 pdfs at once and it took nearly an hour for it too arrive.
I thought it may be Amazon not wanting to get charged a fortune for numerous mail outs etc.