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