batch file string comparison if condition for loop

If condition of the code below does not work; what is wrong with it? The aim of the code is to list names of directories included in a directory, names of which not start with underscore.

code:

@ECHO off
setlocal enabledelayedexpansion
SET "ptka="
SET "ptkb="
FOR /d %%a IN (*) DO (
    SET "ptka=%%a"
    SET "ptkb=!ptka:~0,1!"
    IF NOT "!ptkb!"=="_" ECHO !ptka!>>list.txt :: ("!ptkb!"=="_") part is not working
)