I would suggest considering the purpose of using concept maps, the stage at which they are used and individual differences between students, as there may be an association between a preference for visual thinking and the effectiveness of using concept maps.
The effectiveness of concept mapping may also depend upon the area of mathematics being studied. If the context already facilitates others forms of visual thinking then there may be less need for a concept map approach. For example, solving simulations linear equations can be viewed as the intersection of two straight line graphs.