Serratus anterior stops working?

I am trying to do standing serratus punches (this exercise) to strengthen my serratus anterior, since I have some shoulder instability on my left side. I perform a tactile check by placing my right hand on my left ribs to feel the muscle activate.

For the first few reps, I feel a strong "pop" and contraction of the serratus. However, as the set goes on (around rep 6-8), I notice that even though I am still physically completing the punching motion, the serratus under my hand feels "flat" or soft. It seems like it just stops firing, even though my arm is still moving. This happens faster on my left (non-dominant) hand than my right (dominant) hand.

If I rest for 2 minutes, the activation comes back strong for the next set, but fades again quickly.

Is this normal? If so, what should I do about it - just keep practicing or something else?