I know of instances where officers were having mechanical problems with their issued guns and the departments just assumed they were user induced. One gun that jammed constantly had a bad barrel. Once it was changed, the problem stopped. One officer I know of was seeing all his bullet holes hitting well left during training. He was initially told that, All Glocks shoot to the left. While some people do find that they shoot Glocks a bit left until they get used to a proper grip on the gun, in this case the sights were out of alignment. Adjustment made, problem solved. By the way, never underestimate the effect of improperly adjusted sights, especially on handguns. Small changes often have a huge effect on the accuracy of pistols.
Another handgun I am familiar with was consistently shooting about six inches low. Sometimes this can be corrected by changing the height of the front or rear sights (if they are not otherwise adjustable), but in this case the slide itself was the problem. It was replaced and the gun now shoots exactly to the point of aim. I know of some similar problems that required a new barrel and slide, to assure proper alignment when the two locked into battery. All manufacturers of police service guns stand behind their products and will fix identified problems. Your role is to identify any problems before the guns are used on the street.
Some guns just don't like certain ammo, and that does not mean that either is defective. They are just not compatible. Put another way, some ammo just works better in some guns than others. For example, I have some pistols that are superbly accurate with Speer Gold Dot. I have others that put Winchester Ranger in one hole at 25 yards and shoot patterns, not groups, with Gold Dot. Some folks just assume that if the gun is good and the ammo is good, they should work well together. Don't assume - test! Many agencies have done extensive ammunition testing for accuracy and penetration in various materials, such as ballistic gelatin, window glass, sheet metal, etc. All of that is certainly important. But how many have tested the ammo in their duty guns for function and accuracy? I have seen cases where certain bands and types of ammo shoot low, high, left or right, sometimes by significant distances, from the point of aim. Simply changing to a different brand, bullet weight, or bullet design solved the problem.
Nowadays, with ammunition being so scarce, departments sometimes change brands simply because they have to take what is available. Whenever you change you should run sufficient tests to make sure functioning and accuracy are not compromised. A lot of people are surprised at the amount of variance they find. It is monetarily painful to shoot up what little premium ammo you can find, but you cannot trust that you will get identical results from different brands or types of ammunition. Always test what you carry, even if your range ammo is working fine.
Another important point about ammunition is that you must keep track of the lot numbers. A friend of mine has recently been having problems with misfires. At first we thought it might be his pistol, but we had misfires with the same ammo in other guns. Inspection of the packaging showed that the ammo was all from the same lot. Bad lots do happen. The manufacturers will make good on the ammo and will usually want it back so they can evaluate what went wrong with that particular batch. Rest assured it doesn't happen very often, but when it does, it will affect everyone using that lot, so you need to know who has it. Occasionally you'll see premium ammo being sold off in bulk, with the warning that it is for Training Use Only. That's a clue that there were problems with that lot. Heed the warning.
Obviously, you won't find out any of this unless you are alert to the possibility, however remote, that your equipment may be the problem. With training being reduced because of lack of money, lack of ammunition, or both, the chance of discovering problems through sheer repetition is reduced. Check out any problem and be sure it is not just the tip of the proverbial iceberg. As I've said before, the street is not the place to discover that your life saving equipment is not up to the task.