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The most likely cause is that your capture adapter is not capable enough to pick up the traffic under review; see, for example, the analysis here (https://ask.wireshark.org/question/20865/80211-only-partially-decrypted/#20876)

There are many other reasons that could impact this too, such as

1. Not capturing in monitor mode
2. Capture adapter does not support promiscuous mode
3. On the wrong channel or bonded channels not matching with test traffic
4. Capture system is too far away to pick up highly modulated frames
5. Some defect in the capture system (hardware/firmware/software) that limits full capability
6. A host of other settings that are in use by the network that the monitor mode adapter might not support like LDPC, SGI, etc.