There are several possibilities:
1. If you are using a rifle with a break-over type action the cases may be stretching and full length sizing is required.
2. I know of two bolt action rifles where the chamber was not square with the bolt face and if the used cases were oriented correctly, they would chamber. Otherwise full length resizing was needed.
3. I had a 700 Remington in .17 Rem for which the neck of the chamber was too short to chamber factory (standard length) cases so I chopped about 0.60 off the necks of cases so they would fit.
4. The cases may need neck trimming for length if they have grown too long for a good chamber.
5. The necks of your cases may be too thick for the inside diameter of the neck area of the chamber. If so, outside neck turn or try a different brand of cases. This was a problem with a friends .22-250.
6. Maybe the shoulder area of your chamber is dirty and the cartridges cannot move far enough forward, like a powder kernal got in there.
7. I think with some loading dies it is possible to push the neck back when seating bullets so that the outside of the shoulder will bulge out. If so, the outside diameter of the case shoulder may be larger than the inside diameter of the chamber at that location.
8. After about 7 reloads with some cases for my 7MM STW, just neck sizing, they got too hard to chamber. In this case, full length sizing got them back so they again chamber easily. This was in a 700 Remington action.
9. That's all I can think of but there are probably other possibilities.