I don't really have an answer other than if you get every bid, you're probably too low, and if you don't get any, you're probably too high. If you keep not winning bids with the same people, they're probably just using you as a second or third bid and the job has actually already been awarded.
I always hated bidding jobs. I had enough work from my regular customers, that I usually wouldn't take the time to bid a job, if it was someone I didn't know.