A loose knot and a gap under the collar
The knot should sit snugly against the collar without pressing on your neck. If a gap appears between the knot and the collar after an hour, either the knot was too loose from the start or the collar is too big. In the first case, technique helps: tighten while holding the knot steady rather than yanking both ends. In the second, you need a shirt in the right size.
A knot like a ball under the chin
This usually comes from a thick tie combined with an elaborate knot, or from a collar without room for that much volume. If the collar points lift and splay outwards, go back to a Four-in-Hand or a Half Windsor. The knot should not be as big as possible, only in proportion.
A crooked, twisted knot
Tying the tie on an open collar is often to blame, but just as often the fabric twists during the passes. Practise slowly and keep the wide end flat with every move. On a striped tie or one with a small geometric pattern, any twist is even more visible.
A knot and tie that do not suit the outfit
A thick wool tie with a hefty Windsor looks heavy with a smooth, very formal shirt and a slim jacket. Shiny, slippery silk with a small knot under a wide collar, on the other hand, makes the outfit look unfinished. Sometimes changing the knot is enough; sometimes it is better to change the tie.