Ruben if your character does not go past the first node you obviously have to check the code and the comparisons involved which handle getting new nodes. From a first glance this seems to be stuff like this:

Use draw_point3d to display let's say my.x and vec_next_node.x. You can use draw_line3d too.
Then DEBUG_VAR or draw_num3d (http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=455738#Post455738) to check both the vec_dist and my.max_x values, and draw my.NODE_NEXT.
Oftentimes nodes and such can have different heights compared to where "my.z" is, let's say when the origin of your model is rather low and the path node z values at a too high position. Then xy-dist will get as close as it gets but the z-difference can, because of your gravity code and such, remain too big. If you only have one floor you can set the vec_next_node.z value always to my.z.