If you are fitting a jointed bit, like a D-ring or loose ring snaffle, there may be no wrinkle on the lips at all. That doesn't mean the bit doesn't fit. Take a look inside the horses mouth by lifting the lips and notice where the bit is sitting on the bars. Pay attention to the horses reaction too. You horse might tell you by mouthing the bit, or other behavior, that its not comfortable. If the bit isn't positioned in the mouth evenly, the horse could start carrying its head to one side. Adjust the bridle so the bit sits balanced on both sides.
Fitting the bit might not just be a matter of positioning, but of the size and shape of the bit itself. Horses can have differently shaped mouths, meaning different bits may be more comfortable for some than others. You might have measured your horses mouth and decided to buy a 5 inch bit. But the length of the mouth piece is not the only thing to consider. Mouths come in different sizes and shapes. Just like both my daughter and I have size 10 feet, that doesn't mean we like to wear the same shoes. Measure the length of our feet and we look like same size. But her feet are flat, while I have high arches. She loves her zip up jodpher boots, but I have to have lace ups. We dont like to wear each others boots. .
For example: you might think of a very thick eggbutt snaffle as a very soft comfortable bit, but for a horse with a fleshy tongue and low palate it may be an uncomfortable mouthful. So if youre having trouble getting your horse to carry a bit quietly, consider trying another type of mouthpiece.
It can take a bit of trial and error to find the correct position in the horse's mouth for the bit. Try adjusting the bit according to the where the bit is sitting in the horse's mouth, and try different types of mouthpieces as well.

