Make sure your farm has jungle grass instead of normal grass, otherwise Moonglow cannot spawn there. I couldn't find any info on if it needs to be in the jungle biome, or just in jungle grass, so you may need to build this farm in the jungle as well.
If your jungle is filled with the decorative plants and grasses, there is no room for moonglow to spawn. Destroy all that daily and cut all the trees down to ensure it has somewhere to spawn if you don't feel like building another farm.
If this still does not work, try looking for new surface chests that have either herb bags or can of worms(which can be shimmered into an herb bag), this has a chance to drop moonglow seeds. You may have to get these from a new world.
Once you do get the seeds, make sure to plant them in planter boxes or clay pots so they have somewhere to grow even if there are any glitches preventing them from spawning in your jungle.
Note that it is best to build your farm in the underground jungle so that it doubles as a life fruit and plantera bulb farm in hardmode.