So many posts on this exact same problem. The beautiful thing about installing a ready made shower pan is that it is fast and cheap. But. So much can go wrong.
Often the flex in the pan will cause leaking, often the seal of the shower drain is poorly installed, many times it's just silicone that is holding back the water.
With a fiberglass shower base you need to have a solid floor and you need to make a good drain connection. You should set it into a mortar base so it won't flex and connected it from below. You should also flood test this connection like any shower.
Building a shower with a clamping drain, kerdi drain or line drain is not that hard. Each offer up better building techniques and offer a shower than is not going to leak because you use it.