You are correct! I found same when I first started reloading.
Changes I have made since have greatly improved my reloads in general, including shoulder length.
Some to name a few:
* Changed to Redding Full Length Size dies
* Redding Comp Shell Holders
* Properly lubricating ALL cases
* Cleaning inside of my dies regularly
* I now trim based off shoulder but that has nothing to do with your post
Overall, with above, I have found much more uniformity and consistency with my loads along with shot placement/consistency.
My two cents...
Added:
I am only a few years into reloading so I am still constantly learning myself. One thing I did was I made "Step Notes" and wrote them on cards to follow each and every time I load and stopped doing by memory. Have one for each caliber noting shoulder, over all case length lengths etc - all the specifics per case. I believe this also helped and continues to help me make good consistent reloads.