Difficulty obtaining a UK visa for a Ugandan citizen?

My fiancee and I have applied for a visitors six month visa for her to come and stay with me in the UK, but she was refused the visa, They stated that she had not shown enough social and economic ties to her home country to sufficiently allow them to believe that she would return home after her six month stay. She is unemployed and like probably most Ugandans has no bank account. It was stated on her application that I would be responsible for her air-fare to and from the UK, her upkeep in the time she spends here, and that I would be responsible for her while she is in the country and ensure that she could not work or claim any benefits. I also submitted my last six months bank statements. Can anyone offer me advice on what to do that would get her a visa.