I primed an ESB with about an ounce and a half of sugar, and it came out with faint but noticable carbonation. No head, but you get some bubbles and a faint line of light foam around the edge of the glass. So with 3.5 ounces, you should have SOMETHING. I've only used more than 4 ounces once, and I've had plenty of carbonation.
In your shoes, I would probably pull them out of the fridge and wait a couple weeks. Maybe the temperatures weren't uniform in your storage area, or maybe some had more yeast than others. Whatever the cause, the first couple might have been hero bottles. It's not going to hurt anything to give them a couple more weeks. If it doesn't help, then start thinking about what to do. I'd be very cautious about adding additional priming sugar, simply because you don't know for sure how much unfermented sugar is sitting in there right now.
I personally don't mind low carbonation. My ESB is very low, and I would prefer more, but if I warm it up to proper British beer temperatures, the low carbonation makes it a different, but still good, beer. Not for everyone, though.