How much to follow the guidelines with handwriting/copywork/dictation, etc. with a struggling child?

I have a couple of children that struggle with handwriting, one daughter in particular who is left-handed and has always struggled with writing, including letter reversals sometimes.  My oldest child did really well; he is artistic so I think the neat writing comes easily to him, and his hands are stronger from drawing so much.  He did copywork for years, then moved to transcription, then dictation at the times suggested by SCM.  

I’m wondering what to do with the others that don’t seem ready, although their writing has improved greatly this last year.  Do I keep pressing them forward onto transcription/dictation or just keep doing copywork and lengthen the sections?   I just don’t want to not expect enough from them, but it’s hard not to feel like they are behind.

Hope that made sense, thanks!