As you start getting better, moving further up the leagues and playing against better opponents, you might find they start to change their receiving position. They may become more aggressive and move forward onto the point of the service box.
This means that your standard side wall serve is no longer so effective because from their new position the opponent will be able to step forward and volley the ball before it gets to the side wall. What you need to do now is decide where to hit the serve to still give them problems!
One way of doing that is to hit your serve onto the side wall further forward than normal. This will now make it awkward for them to volley as the ball is on the side wall at the hitting point.
The opponent will now have to step back to their original position and try to volley this serve after the side wall. Not easy from a forward returning position!
What you will find now is that you have a nice little game of chess going. Your opponent will try to anticipate where your serve is going to be and will adjust their starting position accordingly. You must stay one step ahead and always fire that ball in where your opponent will experience difficulty, so you must be aware where they are returning serve from!