Rehab, left Side

I have an old shoulder injury that the last bout lasted almost 8 months.. Suffered some pretty bad atrophy on the entire left side of my upper body.. So far, after about 12 week of working out 5 days per week, the left side is starting to catch up.. but i have noticed my left tricep is lagging? 
 
here's my question..
I can do cable push downs.. ok.. overhead extensions with a rope OK..
 
but when I try to do an overhead, DB extension.. or DB extensions lying on a bench.. similar to skull crushers... 
when I drop the weight behind my head, it's like i have absolutely no power to push it back up?  
 
this is also effecting my bench, shoulder press and almost any other upper body push but the tricep extensions are the one it really shows on..  
 
I'm thinking it might be a weak Brachialis?   Any thought on why it would be so predominant on db extensions?? or any advice to help rehab would be appreciated..
 

I've started doing 1 day per week just working the left arm with light weight by doing the tricep exercises that seem to be the most difficult..
 

There is never enough time to do everything, but there is always enough time to do the most important thing