We have a beautiful 2 year old cocker spaniel, who we love to pieces. He is very well socialised, went to puppy classes when he was younger, is well trained and is so bright. He loves to learn new things, and adores his walks. Our only issue with him is his bizarre reaction to some people.
When we first got him (at 12 weeks old) he would bark and bark at certain people when he first met them, particularly men. We socialised him slowly and consistently and he would always come around before long and be super friendly. On the other hand, he just loved other people right from the word go... there seemed to be no rhyme or reason to it. Recently, he's started growling and barking at certain people that he's known for a long time. My uncle, for instance is a real dog lover, has 2 of his own, and our Charlie just cannot stand him! We thought it may be because he can smell other dogs on him, then he also started doing the same thing with someone else, a female who has no dogs! I just don't understand it at all! I hate the thought that he's making people feel nervous and uneasy when he's around incase he suddenly turns nasty, but I literally do not know what to do!!
I've read a lot of books and articles on cocker spaniels, and general dog behaviour and as a result we have implemented things such as not letting him on the couches anymore. Only stroking and cuddling him when he is sat nicely at our feet. Feeding him after we have eaten, and pretending to eat some of his food before we put it down for him - (can't quite remember the reason for this one now, and it sounds bizarre writing it down!)
When he started this growling lark at my aunty, I gave her some treats to give to him the next time she saw him, and when he walked in she completely ignored him, and then as he approached her nicely she gave him a treat. This seemed to work really well, but once the treats were gone and she went to stroke him, he started growling again!!
Is there something really obvious in his behaviour that I'm missing? He does growl when strangers approach him and hold out their hands to stroke him. I usually tell them not to bother and just ignore him, but he is a handsome pooch, and we do get approached often. He's friendly with other dogs, and is soooo soft with us and people he sees all the time, it's hard to imagine that he'd turn nasty. So is he just nervous and a bit anxious with certain people? What can I do to help him??
I'd just like some practical tips if anyone has any? I'm nervous when I take him out and people stop me to talk to/about him, and I'd certainly never let a child go near him now, but most of the time he loves to approach people, but then growls when they respond to him coming over and sitting at their feet! 
Thanks in advance for any tips, or insight into what I'm missing - I'm looking forward to seeing what people think of his situation.