Your ganache may turn out lumpy if it does not get hot enough to melt the chocolate. Melting white chocolate takes a bit more patience than melting milk chocolate or dark chocolate. Your cream needs to be very hot. If your cream was not hot enough, you can put the entire mixture into a double boiler or a small pot and heat it up. Whisk as it heats until it becomes smooth. You can also microwave the mixture in a heat-proof bowl in 30 second intervals, mixing between each until it becomes smooth.