Once you have 20 scores in your scoring record, your Handicap Index is calculated by averaging your 8 best Score Differentials out of your most recent 20, which means 12 of your last 20 scores are not used in the calculation. If your Handicap Index remains unchanged after posting score(s), it is most likely because the same 8 scores are still being used in your 8 best of 20 calculation. (Rule 5.2, Rules of Handicapping)
As an example, look at this scoring record of an individual below. If this is what your scoring record looks like, then you can expect some changes soon as 4 of the next 5 scores to drop out are being used in the calculation of your Handicap Index.
For more information on the World Handicap System, please visit the USGA FAQ's section which can be found here.
