I play a lot of support and noticed Leona a lot weaker this second half of the season to the point where I stopped picking her.
This has a bit to do with the Locket and Redemption nerfs that happened recently. The problem with Leona similar to Taric, aside from her ult you can see her coming a mile away and her range is very poor. This means unless you are in a good position and your ult is up, you're not going to be catching people (That aren't terrible).
Since you stated that you are in bronze bracket, I doubt this will come into play but the more players you encounter that understand the radius and zones that each character brings alone, it becomes harder and harder to justify a Leona pick over ... say.... Thresh, Brand or even Trundle.
Don't worry about what you build, as long as you build Sightstone and keep wards up, the rest of your items are irrelevant.