tshark: tcp.data cannot always output data

I captured 802.11 trace with both MAC and network layer data.

I use tshark to output the packet detail data

tshark -T fields -e frame.number -e wlan.sa -e wlan.da -e tcp.data

But, for some packets, the tcp.data can output the data content, but not all of them. I cannot figure it out.

I can use wireshark to browse the trace and check the data with no problem at all.