Here is how JoomGrabber with RSS Reader engine works: RSS Reader is an engine to deal with RSS Sources, it has some output fields: [date], [title], [link], [description]; and there is a processor "Get Fulltext" which will get full text from [link]; the problem is: "Get Fulltext" processor can work with about 50% of websites in the world, not every websites.
In the case of yours, it is probably the "Get Fulltext" processor doesn't work with your RSS Source. We can use an alternative method with HTML Parser processor.
See more: How to use HTML Parser processor?
If even HTML Parser processor can't help you then we will need to build a specific processor to get fulltext from your RSS Source, we will need 1 processor for 1 website, you can use that for many pipe to grab from many rss feeds from the same website.