How much does ad spend really affect sales?

I recently doubled my advertising campaign spend since January has been slower for sales. The first two days, sales were exactly the same as they have been…. so I changed my mind and dropped it down to half of what it was originally after that second day…. well on day three and four, my sales jumped (but I had already decreased my ad spend)… however on day five, my sales tanked to a low that I haven’t seen since I began.

Is it possible that whatever algorithm is used by Amazon is on a two-day delay? Or is it just a big coincidence that my ad spend changes had commensurate results two days in arrears for each time I changed things?

Our normal average sales are typically 1250 usd, but January has been closer to 1000 daily average. The boost that occurred two days after increasing ad spend was about 1300 per day, for two days, then 2 days after the reduction my sales plummeted to 600 for the day.

We only sell one product that we produce ourselves. Honestly, from the Amazon keyword metrics, it looks like all of our search results are from organic searches of people looking specifically for our product. So I am not sure if I’m paranoid about the trend of ad spend / sales….. or if it really does matter.