Help! The Plants in the Greenhouse Are Depleted!

This is the first time since I began to have my Sims garden, that the Perfect plants are suddenly depleted. I had my Sim hire a full-time gardener, thinking that would help. Not so far, unless I'm just being my usual impatient self.

Has anyone else dealt with this? Is there a way to nourish the plants? I've tried having my Sims use the manure from their horses to help, but it doesn't even phase the plants.

I am at a loss, here. Short of uprooting everything and tearing down the greenhouse, which I would rather not do, I just don't know how to fix this. I'm not much of a gardener in RL. In fact, I confess I have a brown thumb and am likely a disgrace to my Ancestors who farmed. 

I would greatly appreciate any and all advice in this matter.

Please, and thank you, in advance.

~GalacticGal