You're probably not doing it correctly. You don't just add the offsets. You need to add them in the right place, change the checksum of the registers, change the size of the file, then calculate the command table checksum. Only then can you load it into PBE and save it so that it's a correct ROM file. If you're only taking a minute or two to do this (or skipping any steps), you're doing it wrong.
Answers