Yorkshire Puddings problems, diagnosed

Start with what the puddings actually did. If they stayed flat and pale from the beginning, the strongest suspects are insufficient contact heat, an oven that was not fully hot, too much batter in each cup, or a batter that never had time to rest. If they rose well and then collapsed, the structure probably had not set firmly enough before steam pressure fell — opening the oven early or pulling them before the walls were deeply browned can trigger that branch. If they rose but came out greasy and heavy, the fat may not have been hot enough when the batter hit the tin. Batter temperature matters, but it is not a simple rise-or-fail rule: warmer batter tends to favor height, while colder batter can favor a more pronounced cup shape.