It sounds to me like your junk filter. You may have it set too strongly. Or else, you have accidentally added good files to the junk filter so it has been trained badly. Go to LW's menu bar, Tools>Options>Filters>Junk filter & change the intensity down below 50% (I have mine set to about 35%.) If that doesn't fix it, then use the untrain filter option on the same option window. If these actions don't fix it then please get back to us. Or let us know how it goes anyway.

BTW it's not abnormal for the junk files to suddenly appear for a second & then disappear. Perhaps they really are junk, & your search isn't good enough or just can't find the real files ... if anybody is sharing those particular files.