I have a problem with my old Mossberg 500. The gun doesn't always load the next shell from the magazine. When this happens if I rack the slide hard it will load the next shell. Any ideas on the problem here? :confused:

is it new or well used??

I would estimate that is probably at least 25 years old, so I would classify it as well worn.

I would start by looking at the shell stops (2). These are long spring steel bars on each side of the receiver that hold the shells in the mag tube. Put a shell in the mag and turn the gun upside down so you can see whats happening at the mag tube and pull the slide back. Try to see if the stops move. If they do, then you might suspect the mag spring.

I would recommend a detailed cleaning which would include removing the shell stops and cleaning their grooves in the receiver before I replaced any parts.

If you are not familiar with disassembling and reassembling your gun (or have a book on it), I recommend you take it to a repair shop.

HAAS
I second a good cleaning to include taking the mag tube off! You'll probably fine a very rusty spring and housing(Tube) if it's never been done yet! I finally figure that out after about 10 years of hunting with mine!

I have cleaned the gun extremely well. I have cleaned the shell stops and the grooves they fit in as well as I can. I can find no defects in them and was also wondering about the magazine spring also. I do not know however how to remove the spring.

Haas, it's been over 14 years since I took one apart but I believe you will find the tube screws into the reciever. You'll have to take the slide off and may need a strap wrench.

This could be worth checking out...
I would think that the magazine spring is okay if you can catch a round when you "rack" it hard. To be sure though you can unscew the mag tube and pull out the spring and while holding the ends of the spring pull it to stretch the spring a bit so that when the spring is reinstalled it will have more tension. If this cure it then buy a spring and make sure the tube is clean. Springs are real cheap at NUMERICH.
I think it will be fun to describe the action of this gun pertaining to the magazine stops so hopefully it will shed light on what is happening in your gun:
As you know there are two shell stops. One pivots in its recess and one is bent to be spring loaded. When the slide is pulled to the rear the bolt carrier contacts the pivoting stop and lifts it so that it can catch the shell in front ofthe one that is about to be let go into the lift carrier. When the slide is pulled fully rearward the right hand rail that guides the slide in the receiver contacts the spring loaded shell stop and presses it in. This releases the shell and the spring in the magazine pushes back all of the shells. The rearmost shell is pushed into the lifter and the rest of the shells are captured by the pivoting stop. This by design is timed so if any part wears or bends that could affect performance.
The only possibilities I see that would cause your symptom if it pertains to these parts is that the springloaded stop is bent inward and not protruding out enough from the receiver, The slide is not coming rearward enough unless forced preventing contact between the springloaded stop tab (formed above the shell stop curve) and the slide rail, or these parts are worn down in thickness due to use. These parts are very hard and do not wear easily but can be bent. I wonder if the slide is coming fully rearward. I mean FULLY! The bolt carrier should fully contact the rear surface of the receiver where the bolt that holds the stock on is seen. I bet that if you bend out the spring loaded shell stop just a little bit you will allow it to contact the beveled portion of the slide rail sooner or affect your symptom. Too much and it will hit the rail and not ride on the bevel. These parts are hardened and can break. Take a flashlight and really study the workings of this mechanism and you will see these parts work and probably see that one or more of these parts require replacement.
When you get some wear and age on these guns and a lot of slop starts showing up then when you work the action hard you sort of throw the parts into action and things work but if clearances are right things should work with just easy manipulation. This is a simply designed gun. My 500 12 guage has a lot of years and use on it and despite the critical coments I have heard others give this brand I am surprised how well the parts have held up, aluminum and all! Mine has a very slick action that allows for fast shooting but it sure makes a lot of noise. When a deer hears the rattle of the forearm I know he thinks...."Oh God! It's him!!!"