Persona was designed as an aid to natural contraception. It relies on women having reasonably regular periods so if you don’t, the little computer is unable to accurately predict your day of ovulation and will tell you to play safe and use contraception (i.e. say that you may be ovulating).
There are several reasons why Persona might not indicate ovulation. The first is that you might not ovulate every month. Few women ovulate 13 times a year. It is also possible that you ovulate too late in your cycle for the kit to detect it. Persona is not designed to detect ovulation, only to indicate the time in your cycle when you are definitely not fertile. It is not even 100 per cent accurate at doing this.
Some women enjoy using it, but if it doesn’t work for you I wouldn’t be too upset or think that anything might be wrong. There are better ways of ensuring ovulation is taking place and just because you have a ‘red day’ when you are trying to get pregnant, it does not mean that you should not have sex.