Connecticut Gov. M. Jodi Rell's Charter Oak Health Plan for uninsured adults has gotten off to a slow start as administrators struggle to build up a comprehensive network of doctors and hospitals willing to participate. The three private insurers who have contracted with the state have signed on less than 3,000 primary care providers and only one hospital. Some medical providers say the lower-than-expected reimbursement rates offered under the plan, as well as additional administrative costs, are keeping them from signing on.