There are two possibilities: either your muscles are just getting fatigued, or you are tensing your opposing muscles so they are working against each other.
You can tell which one it is. If you're tired, like you would be on your 15th bench press, you know your muscles are just exhausted. In that case, you just need to rest until your muscles recover.
But if you just feel like you're hitting a wall, and you don't feel exhausted, then your mind is interfering with your body. In that case, you need to do some mental exercise to occupy your mind to get it out of the way of your body. There are a lot of ways to do this, from focusing on your breathing, to saying a mantra, to saying a word when you reach certain critical points in your shot. The whole point is to occupy your conscious mind and get it out of the way of your body so your body can make the shot it knows perfectly well how to do.