Elbows won't extend

I became aware of this problem when I began weightlifting in 2009. I would get called out for not locking out during overhead movements. Coaches and doctors have given me varying opinions, but no real answers or solutions. During the webinar, when discussing hyperextended elbows, coach said it also goes the other way, but did not elaborate. I basically have energy leaking out of my elbows during straight arm movements and any help would be greatly appreciated. So you have a better idea, here is a video. http://youtu.be/Liv061OFTyk

Can you lock your elbows on non-overhead positions like those for plank, dip, or out to side (iron cross)?