What is the version of your Lightroom.
Please create a new catalog and then try with that.
Also please check lightroom.adobe.com, what all images are showing there.
When you sync images it go to lightroom.adobe.com.
Thank you for your answer.
I use the fresh CC version 2015.2.1
I made the catalog in the 'normal' Lightroom app, and then use the web version (lightroom.adobe.com) to continue working on the images and also flag/unflag them. I use the web version for sharing my catalog with the option 'Show only photos that are picked'. It worked well until today.
Now I'm going to make a new catalog and do the same. I'll back soon.
I just check my original album I mentioned, and now it works well. It still looks a little bit strange but it's OK. I have 145 photos in this collection and 58 of them are flaged (picked). When you open the shared web album, and click on the first picture it writes under it '1 of 145', but actually doesn't show all the 145 pictures, but only the 58 picked as I wished. When I close this pictures and click on it again, it write '1 of 58'. I think it's a very minor problem, so I can live with it.
Thank you for your time and help.
You are Welcome Fabee