There are so many things that can cause this.
Rather than taking something to stimulate appetite, I would
try to figure out what is wrong in the first place.
My appetite goes down the toilet from these things mainly:
-liver stress
-overtrained (I sometimes just take however many days off I need until my appetite comes back)
-thyroid issue (need blood work to determine. You can take morning temp with a thermometer and if it is lower than normal, thryroid has slowed down)
-under eating a few days in a row (my body seems to adjust, and my appetite drops, I think it is thyroid slowing down).
-eating the wrong food. (certain foods just slow my whole digestion down, even if it doesn't cause gas or bloating).
If nothing is wrong, then I would start looking into supps like GHRP, B12, and all that other stuff.
I have found that morning cardio on just BCAA's (low intensity) really fires up my appetite for the rest of the day.