I had the exact same problem with an SL-20 where I work.
The problem was chips... remove the lower sheet metal above the conveyor (yes it's a pain in the ass). and have a look around, our lathe was full almost to the top with rusty chips and other debris. After cleaning all this out the sheet metal was freed up and we had full travel of our tailstock again!
Prior to the clean out the advance would slow down and stop around B-9/10!
Give it a try!