Scratch

I'm working on a couple of games right now, and I can't make anything seem like it's taking hits. Example- I'm making a second (and hopefully better) Axelay clone. the enemy is scripted to change costumes so that it changes colors when it gets hit. However, it won't do that! Can someone tell me how to make this work?please?
P.S- if anyone does help, thanks in advance!---------------------------------------------------------
P.S.S- here's a copy of the script i need help on----------------------------------------------------
<when green flag clicked>
<if touching Sprite3>                                    <if touching Sprite5>
<switch to costume2>                                  <switch to costume2>
<wait 0.01 secs>                                           <wait 0.01 secs>
<switch to costume1>                                   <switch to costume1>
--------------------------------                                 ---------------------------------------------------
<if touching Sprite4>                                     <if touching Sprite6
<switch to costume2>                                    <switch to costume2>
<wait 0.01 secs>                                            <wait 0.01 secs>
<switch to costume1>                                    <switch to costume1>
--------------------------------