I checked Amtrak.com for SFA to LOR on 3/19, and no sleeping car options appeared, only coach seats. I also noticed Amsnag showed NA for LOR to SFA on 2/24. Again Amtrak.com showed no sleeping car option.
Amsnag does what is called screen scraping; i.e., it simulates a browser submission and instead of displaying the results on the screen, reads the page into a text file. The information is dug out of the text based on certain key phrases, usually HTML tags. For example, coach fares are located just beyond the "add to cart" image. I forget what it looks for when it comes to sleeper prices; but if it doesn't find them, it assumes it's an all coach train, thus NA.
Note: the format of the Auto Train page is different than that the others, which caused a bug originally. Someone was kind enough to do the QA for me.
As far as why sleepers are not offered on the specific trip you mentioned, you will have to check with Amtrak.