Campaign performance is influenced by many factors, let's look at some situations that may result in low or no spend on campaigns.
MACS Set too Low:
Setting MACS too low for the product category (competitive markets) or for product lifecycle (new to advertising) would result in low bids preventing your product from getting the visibility needed to obtain clicks. Increase your MACS and see if performance starts to improve in 1-2 weeks.
Bids would also be reduced if the delta between campaign ACOS and MACS is too large (over 10%)? Keep this gap small so you do not restrict the bidder or hinder discovery.
Competitive categories have a high number of sellers, giving consumers many product options to choose from. This means there's a high number of sellers bidding on similar keywords, driving up the cost per click. Sometimes, despite a high MACS, your spend will remain low. Try implementing a Bid Floor. If you think you're bids may go $5 for Sponsored Products, request to have this ceiling raised by our support team.
If this is an Auto Campaign, review your listing. Since there are no keywords in auto campaigns, Amazon’s algorithm will scan your listing to determine whether or not to serve your ad. The more relevant information within your description and title, the more opportunity your ad will be placed.
If this is a Manual Campaign, review your keywords. Are the keywords you’re bidding on relevant to your product? Are the keywords too restrictive (all longtail exact matches)? Have you negated too many relevant keywords, hurting your visibility?
Are you SKUs eligible for advertising? Do you have the buybox? Out of Stock?
Daily Campaign Budget too low:
Make sure you’ve set a daily budget in Amazon that aligns with your category CPC and campaign goals. You don’t want to set a budget for $5, if your average CPC is $1.75.