ptx outputs empty file

[jmoss]

In trying to create a permuted index, ptx creates an empty output file.
Processing runs for half an hour, but does not complete.
Input is about 1000 lines of three-column text with tabs as column delimiter.
Sorting is to be performed on the middle column.
I have only the man pages documentation to go by.
The UNIX sort commands works for sorting only on the first word
Any help would be appreciated.
Thanks, Jim Moss.