The only thing I agree is to look in your FCOM. Additionally, you might need more information from your Xponder supplier.
Depending on your manufactor, you might need to put it to ON. AUTO makes your xponder go silent as soon as you are WonW (i.e. on the ground). These are the more older versions.
Maybe you have different types of xponders in your fleet? Check your friendly AI representative, he may assist your company in transforming your fleet with uniform cockpit facilities (depending on contract even for free).
Good luck
HB