The first shell of a sufficiently strong batch of kava will always have it's own unique feeling associated with it, because it's the first transition from sobriety to mild inebriation. I often experience something like you describe here, where the first shell or two have this promising feeling that successive shells will be increasing the feeling you got from the first...but instead it just becomes a dull sedated slog.
There are many variables involved, some are unknown. Firstly, you shouldn't go into kava thinking it should be energetic. In most case, sedation will be the main effect late into a session. Perhaps less so with heady kava vs. heavy kava...but for me, in the end, they become sedating. Some people are wired to actually get turly stimulated from certain varieties, but that just depends on the person. So things like personal genetics, cultivar/chemotype, potency, mood, set & setting, stomach fullness/fasting, pacing of kava consumption and the random uknown variables all play a role in how the experience turns out.
When things go correctly, you can "peak" on kava sometimes, which is hard for some people to achieve than it is for others, and is unpredictable when it will happen. In this case, you have your initial enjoyable buzz from your first shell, then the next couple start sliding into that more dull sedated vibe, but then instead of that just furthering until you sleep, you might have another shell or two, and a meal, then suddenly your ears and mind will warp. You'll feel a flood of kavalactone effects rush through your mind and body, in which you feel the feeling that you enjoyed from your first shell but x5 or x10.
(this may also be achieved by drinking several at once or in a much shorter time span) You wouldn't be 'energetic' per se, but your mind and body will be very pleasantly sedated and stimulated simultaneously. Not energetic, not desiring physical activity, but definitely not sleepy. You will be fully engaged with enjoying the feelings flooding through your self, being very content to sit down, lay back and enjoy them. It might pass in 15-45 minutes...and then it will return to sedation.