Earths core spins slightly faster than its surface. If Mars core is locked with its surface speed because it is more viscous then this may kill the field generation ability.
Internal dynamics are weeeeird.
Wonder if the moon affects this.
If both Mars and Venus have liquid cores and not much in the way of a magnetic field, it seems like there must be more to it, and it's likely to be one (or a combination) of factors that makes Earth unique in the solar system.