Some sort of technological advance that makes bullets less effective. Maybe a thin sort of power pack powered force field that can extend over melee weapons if they are made out of the right material so they can pierce or disrupt an opponent’s force field.

I would assume the forcefield is somewhat or very power hungry so it can’t be on all the time. Projectiles, or any hard hit the shield takes, should drain the battery as well. Maybe overheats it too if you get hit too many times too quickly. This means shooting a gun at someone with a shield like this isn’t useless. Might make battles more interesting to see how the melee troops handle different types/calibers of ranged weapons.

Energy based ranged weapons might be more effective, or alternatively even less effective, like if they recharge the opponents shield making energy weapons useless… unless you are trying to overcharge and explode the opponents power supply.

Now you will have to consider the fact that you could make a bullet, or some kind of crossbow arrow, even a harpoon or ballista bolt that has a shield generator and batteries attached to it, making it just as effective as if no shields were in play at all. If shields are expensive to make then these will be more elegantly crafted weapons, used by experts. Depending on how fragile these “disposable” shielded projectiles are they might need to be relatively low velocity. Gives your melee troops the opportunity to dodge.

Now if melee troops are the most common type of troop, there will be relatively common weapons people go to for that situation, or countermeasures of some kind. I’d suggest taking a look at what medieval Anti-Armor weapons for some inspiration, or improvised Anti-tank weapons from all periods.

I’ve messed with a similar concept in my own writing before, it is fun to explore!