Knowing when your oil cartridge is empty is essential for a great vaping experience. The indicator light on the top of the cartridge is a good indication that there is less vapor left inside, and it's time to discard the oil. You can also check the color of the oil - if it has turned dark brown or black, it's time for a new one. Additionally, you can try the flavor of steam - if it doesn't taste as good as before, the cartridge is probably almost ready.
Refillable cartridges are great for recovering the oil inside, but even single-use cartridges can be recovered. Simply remove the nozzle that separates the tank from the vaporizer and pour it into an empty, refillable cartridge. Be sure to avoid spilling and getting dirty, and also avoid mixing many different types of oil as this could affect the flavor. The easiest way to know if the oil cartridge is empty is to check that it is lighter, when you don't see any liquid when exposed to light and it gives off a more pungent odor than what was initially produced before running out of juice.
If you don't have a full cartridge handy when you need to vape, you can also use a battery with a new atomizer for each puff filled halfway with the juice from the tank that needs to be refilled. When vaping, the oil will move from its original position in the mouthpiece or to some point along the way where there may have been accumulation due to cooling without use. Common problems that may look like a spent cartridge include not having a full contact between battery and cartridge, using the wrong type of screw to tighten the vaporizer's battery cartridges, and using black market products. It's important to note that when you throw away an empty cartridge, there will still be some oil left in the atomizer.
Try unscrewing the cartridge or otherwise manipulating it to make sure that this is not stopping the vaporizer from working as it should. After all, it's definitely cheaper (in terms of time and money) to solve a simple problem than having to replace a cartridge (and work for the money to do it).