The thing about NDB is that it's so easy it's hard! When I was getting my instrument rating my instructor told me that and I thought "that's a big help". As a CFII I can tell you that it's true. There's no easy way to learn NDB's or anything else for that matter. If your stuck on a concept using the material you have now you try some other material. I have a copy of Rod Michado's instrument book that I use sometimes. Some people like the the way he presents material, some don't. Getting some individual instruction from a CFI is a good suggestion. Just make sure it's a CFI that really understands NDB's, not all do. Also keep in mind that many of the questions on the FAA Knowledge Test are made to be tricky and have no practical application to navigating by NDB. Keep at it. If I can understand NDB anyone can.