This is a somewhat common problem in these, both original and in sportered configurations.
The problems are caused by a number or combination of the following and all should be checked. Clues that dictate are what have you done if anything to the rifle? If it's new to you, then you are inheriting someone elses problem and you'll need to check a number of things.
First, check the fit of the triggerguard/magazine to the receiver itself. There should be approximately 1/16th of on inch gap between them when the trigger guard is torqued down. Anything more and you will have a gap that the mag follower will snag in, usually one side or a corner. Too little and you will have contact and that's not good.
To correct any of the problems above, add shims usually under the front screw if no gap, remove tiny amounts of wood in the same area to close up the gap. Wood shrinks over time and area at the front of the trigger guard digs into the wood and changes the dimension.
Second, Many sporters used milled followers vs the later 1903A3 stamped type. They are interchangeable but as the military found, the stamped were more reliable and didn't cause the problem you are having to a large deg. The shape and weight in addition to other things gave them the edge. Many 1903's have the milled and you can change that out and see if that takes care of it.
Third, Sometimes it's simply a fit issue with a squeezed mag fit in a stock, out of spec a little, or some other thing that makes it a bit tight. You can file corners on the follower and smooth those issue out. This was done by armorers in the military all the time, it wasn't just slap it in and work.
Fourth, The mag spring is the last problem. They get out of whack sometimes and need to be replaced. They are inexpensive as are the followers so if you find everything else checks out, then order one of each and change them.
Just to give you an example of WHAT you can run into:
I bought a sporter 03 that a platoon had made up for their retiring colonel. They did a pretty decent job on a stock and a nice job bluing and putting a side mount scope on it. It fed and function nicely. When I took it apart however, I found a brass glob sweated onto the back bottom of the follower. It stuck down 3/16 ths of an inch but still allowed a full mag. I thought what the "heck" and put in a regular on. It would not feed. Closer inspection found they had inlet the trigger guard a hair too far back and it would hang up a follower. They figured they couldn't move it without messing the stock up so they influence the follower with the brass glob. It worked but this is what you inherit sometimes and we wonder why things don't work....
Hope this give you a clue.