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The best thing to do is to spend a lot of time on the bunker/short game range. Another than that the only thing that comes to mind is technique, perhaps working with a club pro could help you identify a problem if there is any.

Sometimes new clubs require a little bit of time to get used to I'm assuming your big bertha set was a cavity back club and wasn't a thin wedge/blade like the vokey-this could be why it's hard to get used to.