802.11 ACK frames sniffing

Hello, World!

In order to improve my understanding of the MAC layer of 802.11, I'm
sniffing the air around me with the help of a Netgear wg511t and
Wireshark.

I was able to identify the main mechanisms as the authentification
(Open System mode), the association and the active scanning w/ Probe
Request / Responses and the Null frames used by STA to tell the AP
about their power status. As well as the Beacon stuff. On some
networks (strangly rare in my neighborhood), I've also seen RTS/CTS
frames.

One think I can't understand (nor find in the standard) is the lack of
Layer 2 ACK frames. I've seen a few ones on one network but they were
used only to acknowledge the AP Probe Responses. I've never seen any
Data frames aknowledged, on that precise network or others in my
neighborhood. Moreover, that's the only network on which I've seen
Probe Response acknowleged. They are not on other networks around me.

Here is my question: Is it because of a misconfiguration of Wireshark?
Is it a hardware problem? Is it a misunderstanding of the standard?

Thanks for your help,
-AJ