Pool losing a lot of water HELP

[Reembeam]

We started filling our pool up all is good until the water reaches just below the skimmers then it starts draining very fast the pump is off the sump skimmers and vac point valves are closed. We don't know why it is doing it it doesn't loose water when it's half full why does it loose it when its full and near enough completely drains then stops. There are 2 Screw in plugs the sump itself one in the floor of the sump and another in the wall of the sump.the in the floor of the sump is screwed in and the one in the wall of the sump is out we don't know what they do if anyone could tell us. And tell us the reason why our pool is losing soo much water very fast. Sorry for the punctuation. Thanks