That pattern is typical for minnows. Pulses of engagement, followed by dry spells. It's a side-effect of Steemit curation. If you are visited by a whale, or someone features your post on a curation thread, you get a lot of likes.
Be careful posting large blocks of work that you intend to sell elsewhere. Most tradpub houses want only original, unpublished work. Even Amazon considers posts on the blockchain as "prior paid publication", which will impact your revenue if you eventually self-publish.
Thank you.