Every sportsbook promotion comes with a page of terms and conditions that almost nobody actually reads in full. Most of that page is boilerplate. A handful of specific lines are not, and skipping past them is the most common way people misjudge what a promotion is actually worth.
Minimum odds requirements
Many promotions require your qualifying bet, and sometimes the bonus bet itself, to be placed at odds above a certain minimum, commonly somewhere around even money or slightly better. A bet placed at odds below that minimum simply will not qualify, no matter how the rest of your strategy is planned. This is one of the most common reasons a promotion does not pay out the way someone expected, not because anything went wrong, but because the qualifying bet did not actually meet a term that was stated clearly in the fine print.