Earnings will vary from day to day and month to month. In my case, my site normally earns a little less on Saturday and Sunday than on other days. I can look at the analytics and see that that is logical because I have fewer visitors on Saturday and Sunday.
In addition, if you are looking at a daily total for the previous 30 days, you should expect differences. This is because when you move that 30 day period forward by one day, you pick up a new day on the latest end of the period and drop off a day on the earliest end of the period. If your new 1st and 30th day have lower earnings than the previous 1st and 30th day, you could see a decrease. Conversely, if those two days are higher than the previous 30 days, you could see an increase. Of course, this is not exact because the days in the middle also vary a bit, although there is probably an average over those 28 days that is reasonably consistent.
No two websites will be the same. But in short, it's better not to obsess about small changes up or down, or even if they don't seem to increase. Increase comes from higher traffic, more time on page, more ads displayed, etc.. Site revenue can vary a lot just based on the topic of the site and the type of traffic the site attracts.
You might look at your analytics and see how long people are staying on your pages. If they are there for very short periods, they may not be there long enough to trigger ad counts. If your site targets countries other than America and some other English speaking countries, your ads may not produce as much income per view.
If you do not have enough ad placeholders, you could be seeing lower income than you could possibly have with more placeholders.
If your pages are short, they often won't produce as much revenue as longer articles because there are fewer ads and time on page is often much shorter.
When our sites are new, it's easy to be bothered by a few cents decrease. This is purely my opinion, but I think it's better if we just focus on producing new and better content that ranks better, attracts more readers, and keeps our readers engaged longer.
I hope this helps.