I need to make sure that you actually randomized the results. Here is the pattern to follow: save while Heather is on the beach, ask Heather to walk on the beach, click compliment Heather, click through the posing scene while not staring at Heather's boobs. If you see that Jessica hasn't shown up and then reload your save, you must change something before asking Heather to walk on the beach to get another random chance. If you simply invite Heather to walk on the beach again without changing your location and returning or even opening your phone, you aren't getting a different random result; you are getting the same result. If you try 100 times without changing the randomizer, then you have looked at one result 100 times.
If this isn't the problem, do both girls have high affection, love and lust?
If that isn't the problem, I've found that sometimes the game has a hidden requirement that we don't know about. Like having to beat Jennifer at poker once before Joy will invite you to play poker.
If none of these are the problem, try again the next day at a different time.
Jessica needn't be present for the event to trigger.
