I recently installed an Eleganza Shower Base and Wall Set in an upstairs bathroom. There was a leak at the drain. Subsequently I discovered there was a missing joist under the shower base subfloor resulting in a span of approximately 15-16 inches of unsupported subfloor, other than the 1/4" underlay on top of the subfloor. The drain was found to be loose, despite my securing it using hammer and screwdriver. The base was attached to wall studs with roofing nails, as per instructions. The shower drain was typical 2" ABS low profile. The weights of the homeowners and son are between 130-260 lbs. Could the levering effect loosen the drain to cause the leak?
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.