My question is pertaining to missing non-Fard prayers such as the Witr Prayer... Is it obligatory for a person practicing such a prayer to make up for it if he/she misses it, for example, by sleeping or just forgetting?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is desirable for the one who has missed a voluntary prayer due to sleep, or forgetfulness, or to being busy, to make it later but it is not mandatory.

Abu Sa’eed al-Khudri narrated that the Prophet said: “If one of you sleeps (past the time of) the Witr prayer, or he forgets it, he should pray it when he remembers it.” [Abu Dawood with a sound chain]

When one makes up his Witr prayer in daytime, he has to perform it in even numbers. If he is habituated to performing three Rak’ahs of Witr, he should perform four Rak’ahs when he makes it up in daytime. If he is used to performing five Rak’ahs he has to make up six Rak’ahs in daytime. But all other voluntary prayers should be made up as they are usually done.

Allah knows best.