You should go ahead and file a claim if the 20-day window is almost at its end. That way, you are protected if the seller did not respond.
A similar situation happened to me last year and because I was in communication with the seller, I figured I did not have to file a dispute through paypal. But the issue was never resolved (the item was never received and the seller will not give me a refund) and by the time I needed to go through paypal, it was too late (past their dispute window). So, definitely escalate your claim before the deadline if the seller haven't gotten back to you yet.