The best things you probably can do is enclose a nice note with your sent package explaining about feedback. You kind of have to be careful when you are chasing feedback because sometimes people get annoyed and leave you less than high stars. Some sellers communicate often through the buy process and leave thier feedback (for buyer) right away. That might encourage the same. In your situation I would send each one the feedback reminder email and if that doesnt work just try a little different approach. More than one request doesnt seem to work out. Might be better with a personal email asking if everything was okay and if it was to please close the transaction by leaving feedback. Good Luck.