In Windows 8, Lightroom 5.6. The search/filter function cuts off the top 160+ images. These are NOT included in the search.
Even when there are as little as 6 images it does not include the first 4 images.
I found this when I was searching a particular ISO. It did not show in the metadata search. I knew there were over 60 images at that ISO.
I tried a text search, it did not work either. The top 160+ images were left out.
Also it renumbered the images. My image number was #163 but Lightroom said it was #1.
In Windows 7 Enterprise, Lightroom 5.6 worked perfectly.
We'll need more detail to figure out what's going on. While we could play twenty questions, the most efficient way to troubleshoot this is to start out by posting a full-resolution screen shot of your LR window.