It could be a number of things, but what comes to mind first is that you’ve got some crud under the shell stop. That’s the bar that stops the shell from coming out of the magazine when you load it.
Looking at the gun from the bottom, it’s on the right side of the receiver. Pump the gun and make sure the stop moves enough to let the shell slide out of the magazine. If it doesn't there may be trash behind it.
If it is gunked up shoot some air or cleaner in the groove it fits in and wash it out. Be careful there’s a small spring behind the shell stop so don’t dig it out while trying to clean it.
If clean it should be easily pushed completely into the groove and the spring should push it back out. Try doing it with your finger.
All the above should be done while the bolt is closed. If this doesn't solve it’s failure to feed at least you have eliminated one potential problem.
Of course this is just an educated guess. If the gun has some age it may be wear and tear on other parts.