By default, RainPro uses a pre-defined cycle of line-weights which are used in turn for each line that you draw. The default is to use ‘fine’ for every series. To reset the cycle:

chSet 'Nib' 'medium,broad'            © or
chSet 'Nib' 0.4 1.2                   © by points

You can set the weights to an exact line width in points, or use the pre-defined weights ‘hairline’, ‘fine’, ‘medium’, ‘broad’. Unless you want to make a really strong emphasis, it is probably best to set all the lines to the same nib weight.