Well first off, your glucose meter should be telling you whether you're getting your dose of Byetta or not! That being said, if you're not, go to the Byetta web site and get the customer service number, call them and tell them you have a defective pen. They will probably arrange an exchange for you at your local pharmacy. As you know, the Byetta pen usually makes 3 clicks as you depress the plunger. If you push it to fast you won't hear three, thats why I push it gently and slowly so that I can get all three clicks. 90% of the time I can feel the actual injection. If your pen isn't defective, you may not be getting that last click so count them as you inject, and push the button slowly so that you can hear each one.
Now, if the aforementioned is not the case, yes, you do have a defective pen and the sooner you get a replacement the happier your blood sugar is going to be!!