If the flights are all showing "0" as inventory the flight is likely to be forecast to be cancelled.
if you are able to book the segments point to point the problem is with the Inventory Management System.
Basicly Delta is not interested in allowing you to occupy a seat on the FLL to ATL, or the ATL to ISP flights for the fare you specify, Delta would rather sell the seats to someone else that is traveling from XXX through Atlanta to FLL. and XXX through Atlanta to ISP that has a higher revenue potential