I do not have an automatic solution but I think that I found what the problem is. I noticed that was is incorrect with the problem sfz files once they are loaded in HighLife is the root note and the note range of the samples.
For instance with one file, all the roots where set to C-2 and all the ranges were from to C-2 to C-2. Which of course resulted in no sound or a very big sound if I played C-2 .
Manually resetting the roots and range made the files play correctly. Files can then be resaved from HighLife. This process is a bit time consuming so a faster or more automatic way of doing this would be welcome.