Why is my News Feed limited?

I dont do a lot with FB, but when I do, I am puzzled that my news feed seems to be limited. There must be a setting somewhere that I dont know about. I have about 20 friends and so I get to see their postings as they come in but apparently not all the time.

I sort of expect that as any new item comes in it would be displayed at the top. And as more new ones come in they would be at the top and the previous ones would be bumped down and so it would be a continuous process, with older ones available if I am willing to scroll down to find them.

But the list keeps getting shortened - ones at the bottom disappear at a rate that I think seems pretty short-term. I would think there would be at least maybe a few days worth, but sometimes I might find only a half dozen or so, and only a day or two's worth.

It's also curious that sometimes I can refresh the page and items will appear that were apparently culled out previously. That is, say I have 10 items showing, I might refresh and suddenly 3 more will appear, but down the list a way, so they came in yesterday and weren't shown when I first opened up, but did show up when I refreshed, even though there were more further down the list that were older all along and they were there at first and stayed there.

I really would like to find some way to say "Show me the last 10 days" or something like that. But I find the settings in FB the damnedest things. I used to be a software engineer and I would never give a user settings that were so hard to figure out.

Sorry to whine, but it bugs me.

Oh well. Thanks for any advice