Remember the basics of what we teach. Chlorine is consumed by either organics in your pool or UV from the sun. Since you eliminate the sun during the OCLT, the only possible explanation for your 1.5 ppm loss is organics (which you don't want) in your pool.. is there something I’m missing???!!!
You must stay the course until the FC kills enough of the organics (and you clean and brush and backwash to get them removed) that you will pass the OCLT. You are close.
Oh NO, @mariane! But... awwwwww ?.Maybe ducks are swimming in your pool.
That happened to me, why the water was clear and failed OCLT. The ducks were having an early morning swim!
If it is ammonia, the pH and TA will drop as chlorine is added.
So, that can help confirm that you have ammonia.
Get your system operational, get the pH to below 8 and begin SLAM.
Monitor TA as you go to see if it drops during the SLAM.
I suspect that you probably have ammonia and hydrogen sulfide.