Regular cleaning and maintenance is important with any vaporizer to keep it working well. In many cases, the best way to clean the oven is with a cotton swab lightly soaked in isopropyl alcohol. You can use the swab to wipe out the inside of the oven, but make sure you hold the unit upside down while doing it so the alcohol doesn't drip down inside the unit.
If there is considerable build up in the oven you may need to do this several times. Make sure when you are done that you turn the unit on high temperature for a full cycle or two until the alcohol smell goes away. If you use it before the alcohol is evaporated off it will taste horrible, so make sure to do the full cycles to burn it off.