Bid strategies need at least 3 weeks of conversion data before they can optimize bids. Be sure that the majority of keywords or product groups in the portfolio you identify has been driving traffic to your conversions for at least 3 weeks.
If there's a significant difference between the time a click occurs and the time a conversion that is attributed to the click occurs (referred to as conversion delay), a bid strategy requires more time (3 weeks plus the conversion delay) to effectively optimize bids. For example, if the average conversion delay for the portfolio is 2 weeks, the bid strategy will require 5 weeks of historical data to optimize bids.
In addition, you'll need more than 20 conversions per week for the bid strategy to be effective. If the conversions in your portfolio don't add up to at least 20 per week, try adding more campaigns or more conversions.