If you check documentation for the getLaunchIntentForPackage(..) method you see
"Returns either a fully-qualified Intent that can be used to launch
the main activity in the package, or null if the package does not
contain such an activity. "
That's pretty self explaining.. your "app package name" is not correct, it might be for some apps but not for others.
try something like
PackageManager pManager = getPackageManager();
List<PackageInfo> packs = pManager.getInstalledPackages(PackageManager.GET_INSTALLED_PACKAGES);
for (PackageInfo pi : packs) {
if(pi.packageName.toLowerCase().contains("app package name") )
{
Intent intent = pManager.getLaunchIntentForPackage(pi.packageName);
if (intent != null)
startActivity(intent);
}
}