When large files are uploaded to the B2 servers they are broken up into smaller parts (using the b2 upload_part call), each part of the file is assigned a SHA-1 checksum. The entire large file itself is not assigned a SHA-1 automatically. This is why you are not seeing SHA-1's for certain files.
If you do know the SHA-1 for the entire large file being uploaded, we do recommend specifying the SHA1 in the fileInfo during the call to b2_start_large_file call.
Inside the fileInfo header for the b2_start_large_file call, specify one of the keys as large_file_sha1 and for the value use a 40 byte hex string representing the SHA1.