The main reason for "not hitting" is the realism. Shooting might feel strange and difficult, but everything makes sense.

For example Mosin Nagant's (rifle) muzzle velocity is about 800 m/s. When you shoot target which is 200 m away from you, bullet travels 0,25 seconds. 0,25 seconds it's not long time, but makes shooting way more difficult. The further the bullet travels, it loses power and bullet falls because of gravity.

These are the reasons why shooting is so hard.

Gravity makes bullets fall. You should naturally aim above the targets. You can also adjust your sight with mouse scroll for further targets. You should watch where the bullets hit, if they hit too low aim higher and vice versa.

Some guns have lower firepower at further targets for example pistols and SMG's. You should aim more higher with them because of the lower firepower.

Shooting moving targets is the most difficult thing. When target moves at distance, you have to aim front of the target and shoot. You have to aim a bit higher also, because bullets will fall. If the enemy is moving close of you, you don't need to shoot front of it.

Why so? Because bullets will travel and they will fall! You should be aware where the bullets hit, do they hit front of the target or behind the target? Or do they hit too high or too low?

With close range guns (SMGs, pistols) you have to shoot more front and higher, because of the lower muzzle velocity!

This thing is for more advanced players. If you see an enemy partially you can try to shoot him through ground or wall. Rifles and MGs are the best "penetrators", SMGs moderate and the pistols worst. Just keep shooting different materials with different guns, you will se how they penetrate. You can only learn this skill by shooting.

Now you probably know a bit more about basic shooting. It's impossible to become good shooter without practising. A good thing is to do is to practise one weapon at time, rifleman is the best class for shooting practises.

I hope that this was helpful! Thanks for reading!