Tractor margin could be 0 even if Tractor Sale amount and Cost has different amounts because for system to calculate margin, there has to be an active rate plan. To avoid such scenarios, please ensure there is a rate plan for each supplier for a customer, in case you do not want to offer discount, system allows you to create a rate plan with "No discount" option which allows system to calculate margin and charge retail to customer.
If you see margin is not calculated (i.e. margin is zero) then please verify that rate plan exists for the fuel supplier (and for truck stop if rate plans are defined individually for truck stops) and for the customer transaction belongs to.