TCP checksum validation (off) doesn’t show my messages

Hey, I wrote a dissector,

Some of my messages are not seen by the wireshark, even though the information is inside the .pcap file, and I can see a specific message that doesn't be dissected as my protocol, as "[TCP segment of a reassembled PDU]".

When I turn on the option in the TCP preferences "Validate the TCP checksum if possible.", I suddenly see the message of my protocol, as my protocol - but with an error of checksum:

Checksum: 0xff30 [incorrect, should be 0x1a23 (maybe caused by "TCP checksum offload"?)]
[Bad Checksum: True]

My question is, Why can't I see messages of my protocol before checking this option, and what is this error about?

Thanks ahead...