If I remember correctly the HESH round can go farther than the sabot or heat rounds, and is favored for this reason. I think it also does not lose penetration over distance due to it being like a heat round.
On the other hand the HESH round is like a heat round and has a long arc, and in SB it is harder to hit with it. Not knowing what the issue it at long range, it would be hard to speculate anymore on what it could be.
Possible solution would be to put max range on and set it to under 4k or 3.5k meters. This is one thing I do not use as much as I should, and probably others as well.
From
http://tanknutdave.com/the-british-challenger-2-main-battle-tank/ High Explosive Squash Head (HESH for short) is a multi-functional round. When it impacts on a wall/armour, the explosive spreads on to the outer surface, known as a “pat” and then is detonated, which sends a shockwave through the armour blowing off an inner section known as a “Spalding”, which is the size of a plate and fly’s around the turret cooking off the ammo, cutting hydraulics and carving the crew up. The blast can also damage the sights, sensors, tracks and main gun inflicting either a firepower kill or mobility kill. Its extremely effective as a demolition round and anti-personnel. Its armed by the rifling of the L30 and requires a low m/v to form the pat, so is fired at a high trajectory to gain range using the L3A3 cordite stick charge.