How can I fix this problem ?

I created a website few months ago and everything is good very well, my ranking are moving up.

But I noticed something which seem odd to me.

Here is an example, when search for my keyword let's say "buy bag", my website will show up in 1 - 12th position in serps.

But when i search for "buy bags" on the other hand, my site shows up on page 10+

Please guy's, what cloud be the problem ?. How can i fix this ?

Hello there,

Nice question, but my friend you must be aware that "buy bag" and "buy bags" are the two different keywords. I know the only difference there is "s" character but its a big difference in the terms of Google algorithm book. If you want the rank for both the keywords, you have to do work for both phrases of your keyword. Using the keyword phrases will help you to come out with this situation. Try to build qualitative links and you must get the rank higher on both the keywords.

single latter make big different

You have to do SEO for this keyword, like @marshasarv already said. Every letter in a keyword makes a lot different for SERPs.

work on both keywords hand bag and hand bags these are 2 different keywords so work on both keyword

It la because both are different keywords. So you must build backlinks for both if you want both to get good ranks

Google will not class both the keywords as same. Both are different keywords in the eyes of search engines. In short, singular and plural version of a word are two separate keywords for SEO rankings. Not to forget the fact that the difference in search volume between a singular and plural keyword can be day and night in some cases.

As a tip: Use both the keywords in your content. Also, work on the landing page.

If you are a retailer, ranking for 'buy bag' is a good thing
For wholesalers, ranking for 'buy bags' would be more beneficial

That said, ranking for both the terms would be great be it wholesaler or retailer.

****WARNING HIGHLY SPECULATIVE ANSWER, MAY NOT FEATURE ANYTHING EVEN CLOSE TO BEING TRUE****

Well u should look at the results that are ranking for "buy bags". Google may be considering the context and the meaning behind "buy bags". For instance when u say "buy bag" u mean buy 1 bag and with "buy bags" u mean buy more than 1 bag.
Google may think "buy bag" is buy 1 bag (handbag, backpack, suitcase???) just like u do, however Google may associate "buy bags" with buying garbage bags or pillow bags all things which are more likely to be used only in multiple form. That depends on which context is more searched and which context satisfies a higher percentage of the people searching. What I would recommend is (if you are not already doing it) add another word (women's, men's, leather, school, laptop, cloth, eco, shopping, vintage) and try to rank for them - "buy women's bags", "buy men's bags", "buy leather bags", "buy cloth shopping bags" if they are beneficial that is.

Good luck !!!

'buy bag' i guess has a small competition and 'buy bags' must be having a huge competition so you should work on making good back-links for it to rank