Two things could've happened here. 1. Not cooled down - The custard won't be set when you take it out the oven. It needs to cool down to harden, so if it was still warm when you cut it, this could be why! Make sure it's at least room temperature or has been in the fridge to cool completely before slicing. 2. Not cooked enough - If it's not set at all even when cooled, it sounds like it's still uncooked. It's salvageable! Pop it back in the oven and follow the instructions in the question above.