I have done it countless times. It has also happened to me too.
Thats because, as it is said, 'You're doing it wrong'. You need to come from a high angle of approach so you can penetrate the top armor. In doing this, you are also avoiding the main gun as it can not traverse all the way up.
you DO know that bumpINg is against the forum rules, right?
6- Members are asked to not act as "back seat moderators". Issues with any breach of rules should be brought to HTC's attention via email at support@hitechcreations.com.