One change worth trying: send the quote by text as well as email. Email sits unread; a text gets read. Same quote, but the text carries one question with it — 'Quote's in your inbox, does the scope look right to you?' People answer a question, they ignore a reminder. Say the expiry date in that same text, plainly. After that, two touches and then one closing text — 'Shall I close this one out for now?' That last one pulls more yeses than you'd expect. Past that, archive it. Chasing a dead quote costs more than the job's worth.
