Well, the lesson horse is no doubt pretty dull to the leg, as is often true of lesson horses. They get ridden by so many different people and so many different skill levels, they must learn to tune it out somewhat , in order to stay sane.
If you are new to riding or only ride once a week, your leg may be weak. But it's definiely a problem of the hrose ignoring your leg aids.
Don't use spurs, use a crop. Use it and use it big, and get a result.
Be ready for the horse to surge forward, and when he does , don't you dare bop him in the mouth for doing what you asked. Be sure to be prepared for that surge and ride it forward.
Its like this: Leg on lightly , leg on bump, bump !!! Crop ONNNN! and then go forward. Cruise forward, and when he slows down, leg on , and if no response, CROP!
Always give the light cue first, but don't wait too long before putting the crop on briskly. You must make an impression that he will remember so that you don't need to nag constantly.