As part of the learning process, children need to have time to form new pathways in the brain to interpret, comprehend and perform new skills. Even as adults you will find that if asked to perform a previously untried action it takes a while for the mind to understand what is being asked of it and then for it to send the appropriate signals to the rest of the body.
Consider what it must be like for a child, particularly young children who are still trying to work out how their bodies work and are still developing their motor skills. It is therefore not surprising that sometimes it may seem as though they are not progressing and even that skills are deteriorating but this is normal while they are experimenting with new concepts. Depending on the child this may take anywhere from a few weeks to several months, but it is important to be patient and supportive during this process so that the child does not lose interest or confidence while developing at their own pace.