I assume it's only a problem if it's a pattern.
It's normal for some episodes to generate a bigger reaction than others, and it's really easy to, as an example be like "Why is nobody reading this update! Don't they know it's my birthday!?" and forget that oh wait, my birthday tends to fall close to or sometimes exactly on the big American Thanksgiving holiday, so the North American readers aren't checking the update because they're awkwardly eating turkey with their families and stuff.
If I haven't hit about 50 likes by the end of update day, I tend to think "Hm... that's under-performing", but I usually don't panic. You should really only be concerned if you have a dip that lasts more than a month or four consecutive updates. That may mean the current story scenario is boring your readers, or that you need to shake up your marketing game. Of course, it may mean people are busy with exams too soo... Honestly so long as your views, subs and likes trend upwards on a yearly basis, you're probably doing okay?
It's natural to have a dropoff over time as readers get busy, lose interest, forget their tapas login (my mum does this regularly), so keeping active readers topped up and shrugging off dead subs is a fact of life. Keep trying new places and approaches with your marketing and if a storyarc is dragging, look for ways to spice it up!
