Importing to Unity

if ur package has corrupt mesh or other assets it may happen sometimes!! is the package created by u?

Its happening to most of my packages

There is a lot of processing involved and it can take a long time. (As long as hours for a typical real-life game.) You'd have to say exactly what you mean by "a long time" and describe the models you are importing. It's one reason you need as new a machine as possible with a lot of ram for 3D!