As others have rightly pointed out feedback is voluntary, and a seller can ony leave positive feedback for a buyer, so in general they will wait for the buyer to leave feedback first, making sure the transaction was a good one, before recoprocating.... if they wish.
Some sellers also wait, and leave feedback in bulk too.
You have given and received roughly the same amount of feedback, just 20 amiss, so that is a very good percentage of sellers replying to you.
