You did not mention what level your gardener is. There has been a display bug since the level 15 rank gardening came out. When you turn level 11 it no longer shows you getting experience as you harvest, but you are actually still getting the experience. If you will check your progress on the gardening rank bar you will see it is increasing even though the display when you harvest is not showing it. If you are below rank 11 and having that problem, I would contact support and report a bug.