As an assistant referee, your positioning is prescribed. In other words, unlike the referee, you cannot choose you stand. You must position yourself in line with the second-last defender or the ball, whichever is closer to the goal line. This sometimes puts you in an awkward situation in which you are not best positioned to observe everything you must observe.
If you have a play approaching your touchline but with a possible offside situation, you must observe the play (potential foul), the touchline (potential throw-in), and the offside line (potential offside). In addition, you must watch the referee to understand what help they need and what they can and cannot see. Consequently, one of the fundamental skills of a good assistant referees is to shift focus from one area of the field to another very quickly but at the right moment. This takes practice. Even experienced referees may struggle or miss something because they were watching something else.
Below, you will find a video of an assistant referee on a youth game demonstrating a great skill of constantly shifting focus. When you are on the line next time, ask yourself if you are as attentive as this assistant referee is.