The dots suffer no diminishing returns. Buffed dots can not be overridden unless using another buffed dot. The best way to reapply the dots is to wait for the dot to completely drop from the target before you reapply it. Since the dot will tick one last time right before it drops.
You shouldn't be missing in the lower levels unless you are fighting monsters 4+ levels compared to you.
Don't apply attacks during Primal immunity phases (when most of them cast their special abilities they are generally immune to all types of attacks for a second or two).