Why does the site fall in positions for keywords?

Question

Google

I promote several sites, on different CMS (Opencart, WordPress), and even on marketplaces (Prom.ua), and in all cases when I try to promote according to an agreed list of words (all words up to 1000 frequency) - I get an initial impulse in about 2 weeks, then the positions rapidly begin to fall beyond 300 (I check in TopViewer up to 300 positions). I don’t understand, since I do everything according to the same checklist (robots, sitemap, meta, unique copyright, etc.), there are occurrences of keywords in the text, I prescribe h1, h2, h3, etc. I used to buy links on Prospero, but I didn’t notice a big difference with them and without them. And I didn’t buy links to the following examples of sites in the screenshots.
With what it can be connected? Is this happening naturally or is there something wrong with the sites or how Google perceives them or something? Or promotion and should last a long time (from several months and beyond)?
Here are some screenshots of removed positions of various sites I promote:
#1

On this site, positions have not moved from the marketplace in a month - why? :WITH

#2

This site is on Opencart - some positions drop out and stupidly do not return

No. 3

This site on WP is a good initial impulse and then stupidly drop out

Thanks in advance for the advice, but I almost despaired :(

Answer 1

If the sites are new, then at the beginning search engines give them a boost, and then the selyavi boost ends.
In general, it makes no sense to look at the results for such a short period of time after 3-6 months, you already have to look
Well, how would external promotion with thematic backlinks also need to be done the same

Answer 2

It seems to me that the point is not in the CMS, but in the methods of promotion. For

initial momentum after about 2 weeks, then positions rapidly begin to fall

Well, a very characteristic sign of bad promotion methods.