The winger was offered a two-year deal by the League Two outfit back in May but has opted to bide his time amid rumours of interest elsewhere, including Premier League Aston Villa.
To date, only Blackpool have come calling with Shrews rejecting their offer.
Simpson opted not to take Humphrey on the club's pre-season training trip to Almeria due the uncertainty hanging over his future, and the Town boss has a simple message for the 20-year-old.
He told BBC Radio Shropshire: "The longer this drags on, the less involvement he'll have in our build-up to the new season.
"I decided not to take him (to Spain) if he wasn't committing himself to us. We want him to sign the deal we offered back in May, and while he's holding back I can't involve him in our pre-season plans."