Some horses are hot and spooky and riders are sometimes nervous of putting leg on as the reaction may be, uhh let's just say out of proportion to the squeeze. But other horses do a fine job convincing us that they NEEEED leg, they can't possibly go forward without leg And they are sneaky about it, they start just needing a little but more leg than the last ride. They don't tend to suddenly stop and say GIMME ALL YOU GOT one day (and if they did I'd honestly suggest getting a vet out sooner than later to investigate the cause of that with you)
It's a little like a dog training observation - many many dogs learn to respond to a final cue in a chain you'll hear "sit, sit, please sit ,,,, Sit SIT' at many a dog park, Is your horse deaf? No, No more deaf than the dog who waits for the yell. But there is not a clear understanding of the cue to go forward (or move off) leg.
It's an easy trap to fall into. If you usually use a 3 on leg pressure , then find a 4 creeping in - then a 5 ... you see where this is going don't you? Soon you'll be puffing and aching and your horse will be slopping along not building or maintaining any muscle at all.
So we've identified the issue - what can we do about it?
The first step of course is to realize the issue. - but once that's done you need to design a plan to correct this.. Traditional solutions might include spurs, a sharp crack with a whip to reinforce leg or a grounds person with a lunge whip to so the same thing. Riding is historically not positive. It is often punitive and about teaching horses to avoid a consequence they don't like rather than choosing to receive one they want. There are many reasons for this (perhaps another blog post for another day) but suffice it to say horses were tools, they were utilitarian and their thoughts and feelings rarely entered into the human brain when problem solving. Some of the classic masters always kept the horse's intellect in mind - and today some sports view the sport as more and less of a partnership.
If you are working towards being more positive checking what your horse understands leg to mean is a good first step. When you are at a relaxed halt what happens when you put a 1 of leg on(on a 10 point scale - where 10 is every ounce of power you have) ? A 3? Do you need to go to 5 before the horse realizes you are talking to them? If so back to the drawing board for you. A gentle bump should make the horse move forward. A 1 might get an ear flick back to see what's next.
If your horse understands that a crop is an aid - not a punishment - you may find it easier to reinforce your light leg with a light cue from the crop - touch and see if a 3 plus a teach does the trick. The warning here is you may want (or need ) to fade that cue for your sport - so don't transfer reliance from leg to stick.
What you are always seeking is to use a light aid and get an immediate appropriate response. Go legs should mean go. You should not work harder than your horse! (Well - at least not physically everyday!)