Novelty might have something to do with it, but the common underlying reason for a child losing interest in a child learning toy is that they have learnt everything that the toy has to teach them. You will need to find a new source of stimulation, but don’t throw that old toy out yet. If it taught a concept or a skill, that can be revisited later and reinforced, but only after a little while.
When you go to the toy store to look for a new child learning toy, your first instinct is to go with one that will likely keep your child occupied for some time. While this should be a concern, your primary one should be choosing a child learning toy that teaches a skill or a concept that is new to your child. This should be in key with your child’s development. The toys you buy your child will need to be constantly updated to ensure that they are learning and developing at the proper rate.