The shape of your biceps, including the peak, is determined by your genetics, not your choice in exercises. You can't influence whereabouts muscle is added to the bicep, you can only influence muscle being added.
Do a search on any forum and you’ll read numerous posts on guys advising to do preacher curls, concentration curls or some other ‘bicep peak’ building exercise. Some will swear that these particular exercises have worked well for them in adding a peak to their biceps – but how on earth could they possibly know!!!?
How could they know that regular bicep curls wouldn’t have resulted in the same outcome? Do these people claim to be running consecutive tests of ‘normal’ bicep exercise followed by a period of ‘peak’ building exercises and noting the difference? I think not. Any difference is subjective and perception only.
The truth is that exercise selection in itself is not going to influence your bicep peak, you’re at the complete mercy of your genetics. However, exercise choice well affect how quickly your biceps grow and so how quickly your see any natural peak. Choose the bicep exercises that work best for you and whatever peak you’ve been dealt by mother nature will appear quicker.
Finally, I don't know your current BF% but if you are carry any excess fat, then your biceps may look flat through lack of definition.