Things seem to be getting a little out of hand in this thread so lets try and stick to the facts and instead of demeaning each other work on helping out OP so they can understand better.
Firstly, as a few of the replies have already linked some great videos that explain some of the mechanics at play; namely these ones in my opinion are a good start:
The main mechanic it seems you are facing is to do with Damage Saturation, that is if you continue to fire at the same location on a ship, you will cease to do significant damage on that space. It is likely your first 2-3 shells performed as intended doing appropriate levels of damage and then your subsequent 3-4 shells were treated under the damage saturation guidelines which saw significantly reduced damage output.
Ships are divided into several sections or compartments, each with their own health pools which as a whole make up your total HP. Within each section, the HP has "thresholds" which depending where the HP is in relation to these thresholds will determine how much a shell will hit for. When the first threshold is passed, the compartment will be locally saturated (indicated by darkening of that area on the ship) and at which point damage will be halfed. When the second threshold is passed, the compartment will be considered fully saturated and further shells will cause at most 10%, there is no visual indication of this.
It should be noted that penetration hits on citadels will damage for the full amount regardless of how saturated the compartment is.
The Wiki also goes further into detail on this mechanic if you would like more detail, this can be found here.
To avoid over-saturating a ship with damage, and suffering from reduced damage outputs, consider spreading out areas of attack on the ship to target different compartments.