If someone is unable to perform the detailed rituals on the specific tithi during Pitru Paksha due to unavoidable circumstances, they can perform a collective Shraddha on Sarva Pitri Amavasya (the last day). Alternatively, they can offer prayers, make donations (especially food) in the name of their ancestors, or perform the rituals later at holy places like Gaya, Varanasi, or Prayagraj.