Answer: Rambam (Korban Pesach 5:1) writes that anyone who didn’t manage to bring the korban pesach on Pesach either because they were tamei or because they were too far away from the beis hamikdash could do so on the 14th of Iyar.
While this doesn’t apply nowadays, R’ Yaakov Emden (Siddur Beis Yaakov, Shaar Hayesod 21) writes that there is a minhag to eat matza on pesach sheni. According to the Piskei Teshuvos (492:n11) and Nitei Gavriel (Pesach 57:9), many chassidim do so together with maror and a cooked egg. Others do so on the following night, instead.
Unfortunately, some people have a tendency to confuse such minhagim with performing the actual mitzva. Thus, R’ Yisrael Weltz (Divrei Yisrael 1:130) suggests that the reason why many who eat matza on pesach sheni do so specifically during the day and not at night is to avoid any issue of bal tosif, adding onto mitzvos. Likewise, R’ Eliezer Waldenberg (Tzitz Eliezer 10:27:10) quotes R’ Malkiel Tannenbaum (Divrei Malkiel 5:104) who writes that he had to urge people to stop saying the beracha of al achilas matza when eating matza on pesach sheni.
R’ Ephraim Greenblatt (Rivevos Ephraim 2:129:39), however, points out that this is a chassidishe minhag, and, according to R’ Chaim Kanievsky, not the mainstream ashkenazi minhag. Likewise, the Chazon Ish himself, did not eat matza on pesach sheni (Nitei Gavriel, Pesach 57:n15). Interestingly, the Vilna Gaon was particular to only eat matza on Pesach (Maaseh Rav 185).
In conclusion, there are different minhagim about eating matza on pesach sheni. If one doesn’t have this minhag they shouldn’t make a point of eating matza then.