I ran into a problem trying to use Towelroot recently when I had an OTA update break root on my phone. Apparently, because the way Towelroot works to gain root access is to use an exploit, Google viewed this as a security issue and provided an update to patch the vulnerability. When my carrier pushed it out, I installed it without thinking, and now I’ve lost root and I can’t use Towelroot to root it again. I have an LG, so I’m not sure if Samsung (or your carrier) pushed out that update, but if you are up to date on all of your updates that may be making it impossible to root by Towelroot. I’ll try to check into it and see if a guy I work with that has an S4 knows anything more specific.