We can imagine a number of examples in which an adult might be placed in an environment that was not conducive to learning. Let us look at a few.
First, imagine that the adult has gone back to college. The adult is much older than all of the other students. That adult might feel very out of place. The other students might find it hard to relate to that adult and might tend to make them feel unaccepted. This would mean that the adult would have a harder time learning because they would not feel comfortable.
Second, imagine an adult on a new job. They are trying to learn how to do the job well. The problem is that there is a lot of pressure being placed on them to get results right away. This means that they do not have the time or the freedom to make mistakes and learn from them. This would not be conducive to learning.
Finally, imagine either a student or an employee. They are in a situation where they are not given much feedback as to how they are doing. The student might be in a class that has no assignments for a long time. The employee might have a boss who does not give feedback. In both cases, the adult will have a hard time learning because they will not get the feedback they need to let them know how they are doing.