tominboise: Your magazine spring may be weak and need replacement - I would call Wolfe Springs and see if they make a replacement or, failing that, call Winchester directly.
In the interim, pull the magazine, disassemble it and clean it thoroughly. Check for burrs or rust spots near to top of the magazine that might impede the floorplate moving all the way to the top. Also, while the magazine is disassembled, stretch out the spring a little to put a bit more tension on the floorplate to bring the shells as far up to the top as is possible.
And work the magazine and action with different brands of ammo to see if any one particular brand may be oversized and catches or hangs up in the mag. M2C and HTH. Mikey.