You are not making a mistake. Sparks are temporary at the start of a relationship. You may have more in common with this new man, and that is why you don't feel a spark. Couples that have a better chance of having a longer and healthier relationship are composed of individuals with similar rather than different traits.
The in love feeling makes you feel a spark and it is largely related to biochemical processes in the body such as release of hormones and brain chemicals ex dopamine, serotonin, oxytocin, adrenaline.
You may also have a need for a challenge that you find with other individuals with whom you feel the spark. But, they may not turn out to be the best partners in the long run.