You don't mention the time frame for your changes, but any changes to a site such as adding titles and descriptions would cause indexing changes and until Google has digested and evaluated those changes the initial situation is that you have lost the position that you had and they haven't yet finished with updating your changes. That can take weeks.

Google describes their process for descriptions:

Incorrect meta description showing on Google search
The most common reason that Google shows a different meta description than the one you provided, is that they think that their snippet is better (for that search, for that user). Google's aim is to help users to get the right content to answer their questions and to solve their problems.

These are their recommendations on description snippets: [developers.google.com...]

These are their recommendations on titles: [developers.google.com...]

This is something that has been going on for years. While the links above do provide suggestions that MIGHT be helpful, g will do what it does regardless of your META. This has been observed in the real world by some/many over the years.

Meanwhile, the shorter the META the better. Avoid padding with keywords. Use the K.I.S.S. method.

Any change takes time to percolate, often triggering undesired reviews by HUMANS which only complicates things.

Something else to consider:

I have NEVER used META since 1996 and snippets appear and MOST of the time they are proper and on target. If you don't make a suggestion g is FORCED to create that snippet from the ACTUAL CONTENT.

Having said that---while I was in the biz of coding for others---I would, at their request, create the very best META possible --- with mixed results in the real world. Sometimes it makes a difference, most times it is a waste of time.

Final thought: The meta is not content, merely a suggestion and g knows the difference. Put the time and money into the content and keep the meta as simple as possible. Usually get better results.

Some of my webpages are not displaying the meta description in the SERP (Search Engine Results Page)

They’re not supposed to. Your meta description text is just a fallback if G### can’t find an appropriate snippet.

With a seo myth around concise meta titles and meta description with 60 characters title and around 155 characters description and with your targeted keywords is real. If your webpage does not have any content and you have invested your research on meta titles and meta description then g must like your titles and description to rank the webpage.

There are many factors which help website to get ranked their page on search. With today's algorithm changes content is king and most of the people focuses to build great content for the website with good baclinks.

If your webpage does not have any content and you have invested your research on meta titles and meta description then g must like your titles and description to rank the webpage.

The opposite is also true: g can simply IGNORE a page with no content.

Just sayin'!

g can simply IGNORE a page with no content.

Some years back, G### redefined its “soft 404”, originally a useful descriptor for an URL that should return a 404 but instead does something stupid like redirect to the root. Now it means any page they deem to have insufficient content. For a while they refused to index one site’s front page (pictures and links, enough for its target audience), so I put in a single sentence--14 words, by actual count--and now they’re happy.

Come to think of it, the meta description on the page in question is longer than 14 words. (But not much. I think it’s bing that sulks and whines about too-short descriptions, though they too can’t be knackered to say what, exactly, they’d like to see.)