Something(s) doesn't/don't add up. It's either you're a bad person picker or your offer stinks.
<em>"The salary offered is market competitive"</em> - Probably not. If it was, you wouldn't lack for good candidates. Where are you based? What do you call competitive? Does the pay package offer a salary and benefits equal or exceed what is typically offered to the top 20% of candidates in that role/industry?
WTF is a <em>"delivery executive"</em>? What is wrong with "delivery courier"/"delivery driver"? Where and how do you advertise and describe the role? Something tells me it differs from the reality of it. Do you take the time to find out in detail why each person quits (exit interview/phone call shortly)? You can't expect to make a good hire in future without rectifying the mistakes you keep repeating.
Pretty sure if you put a properly titled ad (delivery driver not a stupid title wrongly implying managerial role) with the right info (actual pay rate, company/candidate's car, benefits, etc) in the right place (your local craigslist or newspaper classifieds for your city, not on some glorified startup board), you'd have the right people and wouldn't be here.