Before injecting, be sure that you have selected the right body part. At the bottom of the main image in P5, where it shows the figure number and body part, P5 will often switch the body part listed so that it says "Main Camera", or if you were playing with lights or a prop or posing any part of the figure then it lists the name of that light, prop, body part, etc... I've found that the injections work 100% of the time if you first make sure that either the body or the head (depending on which injectors) are selected there first.