That is a great question. There are a few things that need to be looked at before a tire can properly be repaired.
The first thing we usually look at is if you drove on the tire when it did not have any air in it. Driving on a tire with no air in it ruins the sides of the tire. The sides become damaged when they come into contact with the pavement and are pinched under the weight of the car. This causes the rubber liner on the inside of the tire to degrade and crumble apart. This liner helps keep the air inside the tire. If the liner becomes damaged the tire becomes unusable.
If the tire was not driven on we then check to see if the puncture is in a place where the tire can be safely patched. Areas that can not be repaired include the sidewall or edges where the sidewall and tread meet. Those areas are weak points in the tire and can not be repaired