Why can I post in some treads but not others?

Because that thread has been closed. Look carefully you will see a lock on the folder.
 
Hey papalittle, it is not a disgrace getting old.....it's just unhandy as hell..
 
At the bottom left, following the last post in the thread, you usually see a blue bubble with "+Reply to Thread" in it. You may also see "Closed." MUCH easier to see than that padlock.