"One bottle I tried worked one way, but the next bottle seemed very different. Are all bottles made with the same strains / types of plants or could that explain the differences?"
What do you mean by consistency?
Do you get a consistent result, each time you use it?
If not, look at other factors that affect sleep, such as caffeine and alcohol.
Also, sometimes people can develop "tolerance" to a drug or medication.
In that case, you can stop the medication for a few days, and then re-start it.