Shadowpaints -
There are various reasons why chicks die in their shells. The fact that they died around day 10 or so gives us a great hint of 2 possible reason.
Firstly it could be due to a lack of fresh air (oxygen). Make sure your room is well ventilated, but not drafty.
A second possible reason for this could be uneven/incorrect temperature. Make sure the temperature is set at around 100 degrees F (in the case of chicken eggs). Slight variations in temperature will be ok, but make sure it is as accurate as possible.
The final reason could be because of a viral infection in your incubator. Sometime eggs can have bacteria on them that can cause the embryo to die inside of the egg. The only way to really get rid of this would be to replace the incubator.
Hope the info helps! Good luck!