If your ECID from iTunes has no A-F characters, so only numbers (eg. 1234567890), tsschecker will think the value is in DEC, while the itunes value is in X, and it will convert to HEX thinking it is in DEC, resulting in a bad shsh2 file.
So if you have an HEX ECID without A B C D E F characters use UDID Calculator instead or convert the value to dec using this site