it can be a few reasons. what is your speed at? Do you continue to move or hold in one place? are you applying to much pressure? possibly too much friction Kupa Inc. - Home Page has step by step with an efile Young Nails templates/video these might help you also
If there is training offered in your area that go that route also (were I live there isnt alot offered so I have to get what I can). Do you have someone that can help you, do you work on yourself to help you get a feel for using your efile.
Is there a backfill class offered in your area that can help you with using your efile.
Never hold it in the same place, never press too hard, and make sure your bit isn't dull..
There is too much force and friction in one area for too long, causing the enhancement to heat up.. You might even think about getting a new drill, as sometimes they get old and the bit tends to wiggle around and cause even more friction..
And if you don't have training, I do suggest you get training from the manufacture of your e-file..
also check which kind of bit you are using...the arbour sanding bands create the most heat out of them all.Next best is the diamond bit, and actually you can get a lubricating oil to apply to the nail before you use those to reduce heat. ALong with that make sure to check you speed and pressure....if your speed is too low you will tend to apply more pressure. a slightly higher speed and less pressure along with making sure not to file in one spot too long will bit you better success, but definately look into a e-file training class...they are most informative, and you will learn some great techniques to cut down your e-filing time.