Hi,
It really depends on the quality system of the company you are working in, but my opinion is that if CAPAs can not be implemented within the due date you need to explain the reasons why CAPAs can not be completed and also to analyse the risks of not completing the CAPAs. If actions were planned to improove a system or a process you need to analyse what are the risks of not improoving it within the due dates. You can also consider some other actions that can be impelmented. And ofcourse to discuss this with the auditor. The auditor can decide if some extension of the CAPA plan can be accepted based on the risk based analysis you can submit.
Regarding the responsibilities it depends on who is responsible to complete the respective actions. It could be the CRA or the investigator at the site. So you need to have the root cause why the action could not be completed and to have some corrective actions for this as well. For example re-training of CRA.
Hope this helps..
Best regards