Android APK file successfully Signed for Play Store but Why it refuses to Install !!!

Problem:
Hey guys, i was feeling very happy because i successfully build and signed it into Google Play Store, and everything was going on well and Google Market let me to publish it ......, but problem arises at client end. When my friend try to install it, Android Device refuse to install it,

 Solution:
1. How to identify the solution ?
      There is a very simple way to identify the problem by " running logcat ", Go to Android Sdk folder and look for the platform-tools, Now copy and past the path in the command prompt. Now, start open Play Store and keep your eye on the logcat ( pause it as it is downloaded).
      Suggestion:  I suggest before to test for logcat, enable your usb debugging.
     Result: YaaHooo, Problem has been solved ...

2. else if -->  Problem with Application Signature:

     Android Uses the signature keys to verify the applications, to make sure that the game/ applications are coming from the same developer(who initially published it), if signature is changed --> Android is not going to install it.
     Solution: first completely install the pre-installed application even its cache, private folders and other data and then try to install the application --------(how to uninstall->Settings-> Applications -> Manage Application -> Select App/ Game and hit it to uninstall)
     Suggestion:  I suggest you to carefully keep the back-up of your signatures
     Result: YaaHooo, Problem has been solved ...


3. else if -->  Problem with device:
       hey dude, check the specification for your device, Several vendors have defined their own way to download file even from Market Place. for example, HTC Desire is using Cache/ Partition for downloads and have the maximum range of 40MB.
     Result: YaaHooo, Problem has been solved ...


4.  else
        hey guys, i am looking from your side ....