Yes, feedback can come when you least expect it, and then again it never comes when you do at times. My rule used to be to wait two weeks, if nothing heard would send a polite email just inquiring as to if the product was recieved ok, if they were happy, and of course drop a mention about the feedback.
Sometimes it worked, sometimes it didn't, but I don't even worry about that now. I really don't think people are too concerned about buying of members who honly have hundreds of good feedbacks, it's more about the percentage.
If you have over 3o feedbacks with 98% positive and up, then I think you're in pretty safe trading territory. To be honest the buyers on ebay have changed over the years, where as the succes of a seller was build on positive feedback in the early days of ebay, it simply isn't that relevent to a lot of buyers that use ebay now.
Having said that, I would suspect that not many are going to buy of someone with to many negatives, but just keep the positive ratio high and it shouldn't really mater how many respond, in fact some say no news is good news and they can be absolutely right.
Frustrating if you are starting out trying to build feedback, but if they are not leaving it, at least you are not getting negatives : )