SEO or paid traffic?
Weight loss products tend to boom in January, and then relatively slow down thereafter.
Have you thought about promoting health products other than weight loss? Other health niches are evergreen and sell equally well over all 12 months.
Mancar
Yeah, i recently spent like $50 on a bing ad for 3 week diet, got nothing. About 20 clicks/400 IMP. I know I didn't run it for long, and I didnt have a landing page, just used direct linking.