I agree with the person who said if you cant hear anything, pushing the bell further into the arm isnt likely to help any. I generally find just resting it over the brachial without pressure is the best way to go, as pushing down just seems to increase background noise as its hard to push down and keep the bell completely still.
Keep in mind some people are just damn hard to ascultate BP on. Its not always because you are doing something wrong. On these people sometimes it becomes necessary to either do it by palpation, or take extra measures to reduce or eliminate background noise or movement by the patient whenever possible.