Neurons earn rewards from the ballots that are created on their behalf whenever a proposal is created. So it will take at least the maximum proposal voting period number of days before you start seeing ballots created from the merged neuron getting rewarded. In the meantime, fractional rewards should be going to your now empty neurons, which you can recover by merging those neurons once again after this period of time. Please check the numbers again at least one week after merging, to ensure that all of the ballots are now fully representative of the new stake amounts.