I will give you that: poor accuracy linked to ammunition itself is not a gun problem, but I will argue it has to do with it. Certain types of bullets or loads work better in certain guns. Some guns can be easy-going and happily digest all sorts of loads whereas others are finicky and need a certain type of ammunition to give the best results. This is a well-known and accepted fact, and it is good practice to test a few loads in a rifle to find the most accurate one for the needs.
Shooters that require a high level of accuracy often purchase large amounts of ammunition from the same lot, to capitalize on the same production quality. If your usual accurate load misbehaves, it may be that the current lot has some variations that are not working in your gun’s favor. Some firearms have very narrow tolerance in ammunition variation. Checking the boxes will give you the lot number. Check this and see if two different lots give you different accuracy. This may mean finding another load, unfortunately, or at the very least other boxes of the good lot.
There are, however, more severe instances when the ammunition really does poorly in a gun. My Ruger Convertible 357 Magnum/9mm Luger has a recorded average 6 shots grouping of 2.12 inches at 25 yards (scoped and rested on sandbags – I am not that good) with the Winchester 115 grain full metal jacket flat point 9mm NATO target load. Another undisclosed 9mm Luger ammunition of the same weight averages a disturbing 7.52 inches (!!!) for 6 shots groups in the same conditions. This ammunition has very tight manufacturing tolerances and shows a mere 10 fps standard variation in velocity (0.9% of the average velocity). It performs well in semi auto striker guns both in terms of reliability and accuracy. This particular revolver just does not like it at all. There is no fixing this, other than using different ammunition.
If you can’t seem to improve your shooting despite working diligently on your technique, try with another type of ammunition with the antagonistic gun. This issue may be harder to detect in handguns that are usually shot offhand and with lower accuracy expectations than for rifle. Do not be quick to blame the gun or ammunition, but if you clear those two other factors of any wrongdoing, ammo is the next possible suspect for your accuracy woes.