Most of the seeds in the savanna seed mix need a cold moist period (stratification). Ideally, the seeds should be sown between November and late January.
Sometimes, rose verbena will germinate without stratification, but a minimum of two weeks stratification (or ideally 4 to 6 weeks) will greatly increase the number of seeds that germinate. You can do this in the refrigerator by mixing the seeds with some moistened sand or potting soil and chilling them for the appropriate amount of time.