Why does my work feel meaningful sometimes and empty other times?

The meaning comes, then it goes, and you cannot see what drives the cycle. Right now you are in a low stretch, which is a hard place to think clearly from, because when meaning is absent it feels like it was never real. It was real. It will likely come back. What you are in is a season, not a verdict. Meaning at work rarely holds steady, because it depends on many moving things: your energy, your sleep, whether the work is in a stretch of progress or a stretch of grind, what else is happening in your life. The cycle is not a mystery about the work itself so much as the weather of being a person. In a low period, the work has not become meaningless. Your capacity to feel the meaning has just dropped. Rather than trying to force the feeling back, watch the pattern gently. For a few weeks, note when meaning is present and when it fades, and what surrounds each. You may find the low periods follow exhaustion, or isolation, or a certain kind of task. That knowledge is quiet power. And for now, keep doing the work steadily, trusting that the meaning you cannot feel today has not left for good.