Why does the keywords' quality score decrease?
I just wonder why would any keyword's quality score decrease when I face no errors in my landing page, my ads are running well, no campaigns were paused,..... everything is the same as it was.
Also, what should I do when quality score decrease? should I remove those keywords with low quality score? or shall I change their bids, match type and so..?
The prime factor which comes into picture when we talk about Quality Score is the CTR. Also note that Quality Score is a flexible metric and it might vary depending upon how well or how worse your keyword has been performing in the account. It is also dynamic in nature and based on what type of search query has been entered by the user and how relevant it is to your keyword, the Quality Score might fluctuate.
In case you have been noticing a decline in your keyword's Quality Score, you should first of all review the CTR, search queries being triggered by that keyword and based on that you should optimize your account. It can be changing the bid, changing the keyword match type or adding the negative keywords to improve the relevancy.
There is a famous old Quality Score Recipe thread by Googler Tanmay and I would definitely suggest you to read that for once: https://www.en.adwords-community.com/t5/Basics-for-New-Advertisers/Ingredients-of-the-Quality-Score-...
Hope that helps!
Pankaj has given you a nice rounded answer here. What you do need to realise is that over time, without any changes QS will in most cases begin to decline.
This is not because your account is getting worse but mostly due to other accounts getting better. You need to be constantly testing and improving your account to stay ahead of the curve. If you allow your account to stagnate then your competitors will improve their ads, narrow their targeting, come out with new specials etc, which will inevitably lead to your ads being clicked on less and theirs more. This lower CTR will lead to a lower QS.
This is not the only case in which your QS will decline over time given no changes but it is a likely example of what can happen.
QS is constantly changing and being updated. In fact the QS of your keyword is calculated every time it enters an auction, so you need to be aware of this.
In short, there is no 'happy medium' you can achieve in AdWords, you are either improving or declining.
Hope this helps