Why do the Walking Dead Zombies only show intelligence in the very first episode?

I enjoy The Walking Dead, however one question keeps nagging me. The 'Walkers' are always shown to not have any intelligence or memory of having been human. This is for all seasons except the very first one.

In the very first scene of Season One, Episode One, we see a little zombie girl stop to deliberately pick up a teddy bear from the ground.

Later in the same episode we see a zombie apparently try to turn the door knob to get into Morgen Jones' house.

Also at Atlanta they can climb fences and used rocks to break glass.

After season 1 I do not recall seeing any such intelligence demonstrated by a zombie again.

My question is: are these initial displays of intelligence a discrepancy in the story, or is there a plausible explanation why the Walkers performed these actions?

Answer

Zombies tend to follow instinct, but have been known to react to certain stimuli that do not involve food.

George Romero is responsible for the typical 'zombie rules', and his original trilogy set those rules. Zombies are slow and essentially mindless, they crave living flesh and prefer human, zombies are drawn to sounds and motion, zombies do not attack other zombies, zombie bites are fatal, and dead people rise as zombies (often changed to 'dead from zombie bites').

Even in Romero's original trilogy, he broke these rules whenever it was needed to make a better movie, as I've said before.

But there are some actions which are so automatic that they become instinctive. For instance, many people who drive the same route frequently don't need to focus on it. They'll frequently have their minds on other subjects, yet still drive safely and on the correct route. It's possible that different zombies retain different 'learned instincts'. Remember Bub from Day of the Dead? He was an outstanding example of a zombie retaining some learned instincts, even before the training he was given.

Further, in the 'Day by Day Armageddon' series the protagonist comes up with a theory of what he calls 'ten-percenters', noting that about 1 out of every 10 zombies seems slightly more capable than the rest. They retain some problem solving skills, can recognize patterns they've seen frequently, are better able to climb stairs or ladders, or something similar that sets them apart.

In a town that's been abandoned as long as Rick's home town, only the most capable zombies will still be present and successful in hunting. Thus, the ones we see that are most active are likely the highest-functioning.

Later in the series, when we're in areas with higher (or recently higher) populations, the most capable zombies just don't stand out as much (or got to the harder-to-reach, better defended 'food' first and have been head-shot already).