How is the site manager handling inquiries? Is your marketing effective at bringing in prospects? Are the units being shown adequately prepared? What is being done to retain good tenants? Are service requests and other issues handled promptly?
Often, site managers ARE the source of your problems. Since they live there, they make friends with some tenants, enemies of others...basic personality issues. You lose the " business" context, and they tend not to treat all fairly and equally. They can become territorial, and sometimes treat prospects as an interruption of their work...rather than the source of their paycheck! They are frequently poor time managers, and have difficulty prioritizing effectively. As a result, they can become overwhelmed with the amount of work and time pressures of a large property.
Finally, there is always the question of honesty. Checks and balances are very difficult with an on-site manager. Do you KNOW your units are actually vacant?