problem calibrating SEN0161 (adjusting gain potential device)

Hi, 

I have two ph sensors (SEN0161). I followed the instruction given in the wiki (https://wiki.dfrobot.com/PH_meter_SKU__SEN0161_#target_5).  Everything works well until step 3. 

When I have to do step 4 “Put the pH electrode into the pH standard solution whose value is 4.00.Then wait about one minute, adjust the gain potential device, let the value stabilise at around 4.00.At this time, the acidic calibration has been completed and you can measure the pH value of an acidic solution.”  it does not work. I cleaned PH electrode and put it in a PH4 solution, waited for more than a minute, adjusted the gain potential device and nothing happens. Usually the numbers will go up, instead of down (initially at around 5, then more than six, for the second one it was around 6 then in move to more than 7). 

I tried adjusting by turning first to one direction, waiting for more than a minute, then to the other direction waiting again… the PH value is alway around 6 / 7, sometimes it will go higher. The small screw turns, but it does not go up or down. Is that normal ? What should I do ?

Thank you for your help !

p.s In the beginning I wanted to do a “software calibration”, a pdf with the code was available in the website where I bought the device, however it has several errors and I could not manage to fix all of them. This is the link to the pdf, if someone has the code, please, maybe it could help.  https://www.farnell.com/datasheets/4376670.pdf?_gl=1*17gy661*_gcl_au*MjA4NDM5ODcyNi4xNzQ0NjI1MzQw