Delaying the prayer off can be an excuse for only two things - sleeping and forgetting. Even then, as soon as one has to perform namaz as soon as possible.
However, if other evidence and documents are carefully studied, there will be more excuses. It is permissible to delay the prayer even if the enemy threatens hard. On the day of ''Khandaq'' the polytheists forced the Prophet peace be upon him to delay the prayer four times.
Also, the midwife and the mother have the excuse when they fear the death of the baby.
Other evidences that a person who has not performed the prayer in time must make the qada. The prayer of qada is done like a regular prayer. If he does qada in the livingplace he perforns completely, if while safar (traveling) he performs namaz short (qasr). The qada of the witr prayer is also performed.
If a person has not made namaz for up to six times he performs his prayers in order. If it is more, the hukm will be disrupted and he performs namaz as much as possible he can.
Those who do not know the number of prayers of qada will pray until they think that their debt prayers are over.