Hi Julia
What electrolysis machine are you using? If the hair is quite strong and thick on the neck area then a size 3 probe should be fine. Try not to feel bad about the bruising, I'm sure it's only small and your client may not have noticed. This would usually be down to incorrect insertion or not stretching the skin enough. As Hols_21 said, get the client to move their head as far to the left or right as they feel comfortable, this should stretch the skin more. Don't be scared to ask them to move about as ultimately you want to be able to do the most effective treatment possible, and sometimes you need a bit of help from your client with those hard to reach areas.
Sam x