What is one limitation of a CPR mannequin?

The chest cannot rise and fall when CPR is performed on a mannequin.

Explanation:

In CPR training, one of the main necessities for performing CPR is letting the chest rise and fall.

The reason this detail is important is so you can make sure the victim's lungs are working properly and to see to it that you're completing the resuscitation procedure properly.  

If the chest doesn't rise and fall, it can lead to even more severe problems because first, you may be doing it wrong, or second, there may be a rip or tear in the lung organ or the windpipes.

they deliver oxygen to the blood

equal distribution i think.