I have a small lemon tree planted from seed. The tree now has a lot of fruits. The first fruit I harvested is not juicy enough. It is getting a lot of sunlight and I fertilize regularly with organic UREA.
What Fertilizer to Make My Lemons Juicy
Certified GKH Gardening Expert
It sounds like the tree needs more water. I would recommend increasing water, particularly while the tree is in fruit.
This article is about oranges, but they are very similar to lemons and the issues are the same. This article will point out some other possibilities: