you should always full size resize when using brass from another rifle
This does not always work. Modern rifles seem to have chambers with less tolerance and if you are using old brass that has hardened, when the case comes out of the sizing die it will spring back, just enough to prevent the full length sized case from going into the new chamber.
The solution is to anneal the old case before resizing it.
Annealing is never a bad idea and had the OP annealed his cases, the problem would most likely have disappeared, saving him a lot of work and worry.
I have personally had two new rifles in two different calibres, one of which was a 243, that acted just like the OP describes and annealing cured each one.




















































