Int his game you have the chance of getting Mutated Berries. Here, if you have different berries grow next to eachoth, **there is a chance that the berries will cross-pollunate** and, within the yeild of the Berries, you will find one or more of another berry. This chance is increased via the use of Surprise Mulch. We'll be providing details on what berries result in what berries as time passes

You're doing everything right, there just isn't a 100% chance that you will get a Kelpsy Berry. You could try using some Surprise Mulch to increase the chance of getting one.

http://www.serebii.net/xy/berryfields.shtml