Dear FSL experts

I have a question regarding the symmetrical tracking in probtrackx2. I have two ROIs: one is a subcortical volumetric mask (sub_vol_mask) and another is a cortical surface-based mask (cor_surf_mask). Now I want to perform a symmetrical tracking between the two ROIs. I first performed probtrackx2 without any option, just put the two into a seed mask text file and got some results that are quite reasonable to me (run1). I then tracked them using --omatrix1 option (run2) and I finally ran probtrackx2 again using "--network" option without "--omatrix1" (run3). I got the following results:

(1) run1: a pathway seemly connecting the two ROIs, with only *one* waytotal number.
(2) run2: a pathway and a waytotal *identical* to that in run1, but with extra files fdt_matrix1.dot.
(3) run3: an empty pathway file and a waytotal with two zeros in it.

I am a little confused and don't know why I got the result in run3, as I read previous posts saying the "--network" is the correct flag for such analyses. I wonder whether other users have reported similar issues.

Any information will be greatly appreciated!

Longchuan