Sperm can live inside you for up to five days.
Having sex every other day is usually enough to get pregnant within a few months of trying unless one of you has a fertility issue. If you try for a year without luck, you could consult a fertility specialist for testing.
Even when everything happens at the right time - something sometimes goes wrong and even a fertilized egg doesn't always attach to the uterine wall to begin the actual pregnancy. During any cycle there is about a 25% of becoming pregnant.
Personally, I had extremely irregular and long cycles. It was absolutely impossible to predict when I might be ovulating, but my husband and I wanted children. Anytime I was not actually having a period, we had sex at least once every three days. We have two children.
Apps are based on past data. Ovulation can be early or late during any cycle for any number of reasons. So, it is always best to just have sex often during the week before ovulation and on the day of predicted ovulation.
If you are still having bad luck, you could also learn how to track your basal body temperature. When done correctly, it is the best prediction of ovulation. HOWEVER, the other problem is that once ovulation actually does happen, the egg only lives for 12 to 24 hours - which is another reason that sex BEFORE predicted ovulation is usually better than just trying for sex on the day of ovulation.