Can a plug in a tracheotomy cause a person to retain co2 in her body even if she is on a ventilator?

Yes because her body still has to clear C02. It means that the machine is placing to 02 in the lungs, however the gas exchange still has to take place, and get the C02 out, a plug can not only cause a problem for C02 out but also O2 in. May need a respiratory tech to come out, or call the nurse.