Try this code to post video on instagram it works for me hope it works for you also
first you have to save Video To Cameraroll then use this
NSString *strURL = [NSString stringWithFormat:@"instagram://library?AssetPath=%@&InstagramCaption=%@",yourfilepath,yourCaption];
NSString* webStringURL = [strURL stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSURL* instagramURL = [NSURL URLWithString:webStringURL];
if ([[UIApplication sharedApplication] canOpenURL:instagramURL])
{
[[UIApplication sharedApplication] openURL:instagramURL];
}