If you can't get the temp to 105C use a different hot plate, or turn the one you have up a little hotter.
If you are using a coffee carafe to do this, and the carafe has a small bottom to sit on the warmer but is larger in diameter than the bottom, and has a top opening as large or larger than the bottom, you may have more evaporative cooling taking place than you have heat input. Up the heat, or use Or use a beaker or carafe with a smaller top than bottom. whistler teapots are recommended if you don't have a beaker or large Erlenmeyer flask.