These are some common reasons why wards, even powerful ones, fail at times. I have noticed these over the years, through my mistakes and others.
…The Ceiling. Did you remember to ward your ceiling? It doesn’t matter how powerful the walls are if an entity can just jump over it.
…The Floor. Also doesn’t matter how strong the walls are if the entity can just dig under it.
A Circle Doesn’t Fit Into A Square: Does the shape of the ward match the room you are in? While a circle is the most stable when standalone, not clinging to anything else, it’s still weaker than something tied to something physical- such as the walls, floors, and ceilings of your space. Thus, even the best spherical ward will still fade out much quicker than something tied to the physical structure of your house. Additionally, having a circle in your (most likely) square-ish shaped room will leave certain corners without protection; you want the intruder to be kept out of your house in the first place, not just certain parts of it.
The Corners, Edges, Etc: Building on the previous point, even when the ward is tied to the physical structure of the house, many people don’t pay attention to the corners or edges where the ward bends- this is often where the ward is usually weakest. You want to double check that the ward is solid on those portions and maybe even visualize something tougher holding it together there, as the bending usually makes it more fragile near those certain points.