Sheila, whatever you do you mustn't blame yourself, you have NOT let Charlie down, you are a great parront! Sometimes unfortunately it is just the case that AG's pluck, they are skittish by nature. I do feel that with Charlie it is just a process of elimination be that allergy or psycological, something somehow is bothering him. It is probably something quite simple that you have taken no notice of. Isn't this what my problem with Argyle turned out to be in the end?? I was pulling my hair out too.
I hope you get some answers from the specialist Sheila but continue to search your mind for something in your household or routine or even Cracker that has caused this unease with him. I wish you luck.