Lightroom CC keeps deleting and downloading pictures

So I transitioned to Adobe Lightroom CC at the beginning of the year and keep having a problem with my 17000 photos cloud: very often when I close the program and start it again, I will find that none of the pictures from my cloud are still there. At that point Lightroom CC starts downloading them again until complete. This is very annoying because not having a very fast internet connection, it takes a while to get them back. I ticked the option to keep a copy of smart previews. How do I solve this problem?

Moreover, sometimes when I upload large numbers of pictures from an SD card or even the hard drive itself, the catalog will become corrupt and I then have to delete it from the local folder in order to reset it. Can anyone help me on this matter? Thank you