If these potential customers aren't having any issues with finding trucks for their freight, there is no problem for you to help them solve. If they have a direct relationship with the carrier's they use, adding a "middle man" will either add to their shipping cost, or cut the rate paid to the carrier. Cut the rate and the carrier may not continue to provide reliable service...which creates problems. Increasing their shipping cost is a non-starter, since they don't want to pay more than they have to in order to move their freight. That's the basic problem you're having, right? Nobody wants to give you a piece of the pie unless and until you can convince them they will see positive results...a good ROI for what you are asking them to pay you to do. If it ain't broke, you aren't likely to find many willing to try to let you fix it.
We have tried to tell wanna-bes it ain't as easy as it sounds. You have to find a niche ( customer ) that NEEDS your services, not WANTS you.
To the OP, in my opinion, brokerage is a dying profession, with all of the technology at a persons fingertips AND more importantly with the bottom line always shrinking at some point in time I can see no reason why any company would even need a broker?