Greetings!
A few weapon-modifications providing certain weapons with devastating armour-piercing or armour-crushing properties can serve to open up heavily armoured Player Characters like cans of Tuna being popped with a sledge hammer. War Mattocks, Heavy Maces, Flanged Maces, and War Hammers do very solid work against heavily-armoured opponents.
That is one way to crack that problem, such as it is.
I also restrict the availability of Half Plate and Full Plate Armour. Most Warriors or Fighters, even running into higher levels, have only a 16, 17, or 18 AC. That also doesn't account for magical armour though. Keep AC from getting crazy, and even higher level Characters are again, threatened even by 4th, 5th, or 6th level opponents. Add in a favoured Critical Strike Table, and the threat of even lower level opponents, such as level 2, 3, or 4th level Goblins or Orcs also maintain a formidable threat ability.
Semper Fidelis,
SHARK