It has worked fine all summer exept it seemed to take a long while to finally cool down inside. Yesterday it started not cooling at all. It's blowing air but not cool air. Checked outside at the cooling unit and noticed the cooling line was all frosted up in places where there is no rubber insulation on it. Is this a sign that it is low on coolant or do I have some other problem? Thanks for your help. R.H. in Okla.