I can’t claim to be an expert, since I haven't raised a large number from flask, but genetics are only going to be one component. Even with long-petaled parents there are going to be a range of outcomes. The other large component with the long-petal species like Philippinense and Sanderianum is going to be water and humidity. Without high humidity and good, even moisture the petals can stop growing. So even a plant that bloomed previously with very long petals could bloom with short ones if it dries out during bud/flower development. I’ve had my Philly parked next to the humidifier while the buds opened up, and I constantly check it to make sure it doesn’t dry out (it’s in a very sunny, south-facing window). I’m not sure what your conditions are, but I find that growing indoors can present challenges to getting the same flower quality as greenhouse growers, even on the same plant!