Answer

This is a feature which is typical of agar media.
 Although it is a gel at room temperature and solidifies at around 30-40°C, it will remain firm at temperatures as high as 65°C or so and will often not melt again fully until the temperature reaches 85°C to 95oC (depending on type). This property is known as hysteresis.
 What you will probably need to do is to bring the solidified agar up to 85-95oC then allow it cool to 50oC, maintaining it there, at which temperature it should remain liquid. All this can be done in a water bath or 2 baths at different temperatures (or heat in a pan of water until it melts, then maintain at 50oC in a water bath).