Can you post a photo as that might help spot where the problem is.
Here’s how I troubleshoot this kind of problem:
1: notches missing or not matching at all?
– I’m trying to attach something upside down
2: centre back and shoulder notches good, one front ok, but all the length missing on the other side?
– I’ve mucked up one placket somehow
3: centre back and shoulder notches good and both sides equally short?
– I’ve mucked up both plackets somehow
4: no notches matching very well anywhere?
– I’ve traced off the wrong size
If you can work through that kind of list then you can zone in on the error. I’ve made the size 10 classic shirt, and there’s no pattern issue. Collars always require easing into necklines, it’s what makes them stand up, but it should be achievable and the notches align neatly.
The trimming off of the extra placket width seems to catch a lot of people out, so maybe go back and check that step.