AI models have inherent randomness in how they select sources. A source might be used one day and completely absent the next — this is totally normal and nothing to worry about.
Don't focus on daily fluctuations. Instead, look at trends over multiple days or weeks, or use tags to see patterns across similar prompts. Daily fluctuations for individual prompts are completely random and don't indicate anything meaningful.
Focus on what matters: Important and relevant sources will be used repeatedly over time. Concentrate on getting your brand mentioned prominently on those consistently-used sources rather than worrying about day-to-day variations.