Agents all carry Errors and Omissions (E&O) coverage, which will reimburse the insurance company if the insurer has to provide coverage for something they otherwise would not have, due to an agent's screw-up. However, crappy agents know that insurers rarely pursue this because they don't want the agent to start badmouthing/not quoting the insurer. As a result, the insurer usually eats the loss, and the crappy agent goes right on being a crappy agent. There are plenty of good agents out there, but there are enough bad ones that you really have to do your homework.
In my case I had been with that agent and company for years and years. Given the circumstances I felt like I had no recourse but to take my business elsewhere. The theft of the trailer was a good 15 years ago or so, and I've never had another car trailer since. Such is life...
