Put an expiry on your offer
An offer with no deadline is a free option for the seller. They will use it to go back to every other buyer and say they have something in hand. Your number becomes the floor other people bid from.
Give it a genuine expiry. Twenty-four to forty-eight hours is normal and nobody will think you are being difficult. It forces a decision instead of a shopping trip.
This offer is open until 5pm Wednesday. After that I'll keep looking.
Only do this if you mean it. If you extend twice, the deadline stops meaning anything and so do you.