I have heard that hacked flowers don't produce offspring. I'm not actually sure if that's true or not, but if it is, that could possibly explain why you aren't getting hybrids from flowers you bought from another player.
Another thing that really helps when trying to get hybrids is to make sure you
only water the flowers you want to breed. If you water too many flowers in town, your daily allotment of hybrids can go anywhere. If you only water just your blue roses and purple pansies though, you'd be far more likely to get a hybrid from them. Villagers watering flowers will also affect this. When I'm breeding hybrids, I start up a new day, quickly water my flowers, and then TT to the next day straight away.