ANA testing fluctuates for some reason between positive and negative results. All of the lupus sites mention this.
The reasons for this are not understood at this time.

If you are taking the older version of ALA...the racemic mixture, you will have to take that on an empty stomach. This version is very slowly absorbed, and presence of food will change how much you absorb compared to an empty stomach.

Also ALA is carried around on a transporter, that also has other jobs. So it is possible your transporter is "busy" and not delivering the ALA the same every day.

As you can see with the ANA testing, things do change and cycle in the body. Why there can be an adjustment to response to your ALA, remains unclear. You could try a higher dose, and see if your gains come back. Or you can discontinue for 2 weeks and restart it. That might show some hints for you as to what is going on. Nutrients are used all over the body, and who knows what priorities are in place to distribute them? Could be your ALA has awakened some other system and is going there for a while?