I'm a lyric baritone. My lowest modal note is D2 (I just developed it few months ago). My higher notes however seem to pose a problem. I can sing an F#4 in just about any day, however, my G4 seems very tough in some days while in other days (when singing a harmony), I've been able to hit an A#4. There was a day in particular that I had an F6 in my falsetto. Since that day, I've never beeen able to reach it again. The highest I get is an E6 (on some days as well). Could this variations each day be due to stress, wrong technique or it's normal. Cause it's quite disappointing when I have to sing a solo for my choir and can't get above F#4. Any tips please?
Many factors can affect this. Low-grade inflammation is among them. If you suffer from respiratory allergies, even mild enough that you notice no other symptoms, you may find that the higher notes become difficult or unavailable. Even lack of sleep can affect your range.
One thing to be careful of is gastric acid reflux. Singers may be especially vulnerable to this because it is common to eat in the evening after a performance, and having a full stomach increases the risk. The acid irritates the vocal folds, which causes inflammation. The inflammation can affect you range. Chronic reflux can also lead to more significant damage to the vocal folds or the esophagus.
At least for me, vocal warm-ups such as singing ascending scales or patterns temporarily increase my vocal range upward. I believe those warm-ups would also work for you (and choral teachers keep making students use them in-class for this reason), so yes, I'd chalk these variations up to wrong technique to a certain extent.