Whenever I dont get feedback in a reasonable amount of time, I send the buyer a courteous message just basically saying "hey the tracker shows your item was delivered X days ago. let me know if it arrived safely. if you have any questions, concerns or comments, please let me know. and please leave feedback at your earliest convenience."
i usually get feedback after that. ive done that since i started selling and ive received feedback in 81 out of 92 transactions.