tar not working as advertised

[HCART at VAX.OXFORD.AC.UK]

    I am backing up using tar(1) on a 3130.  If I give specific files
to be backed up, everything functions fine.  But if, to back up all
files changed in the last week, I use

  find / -local ! -type d -mtime -7 -type f -print | tar ca-

as shown in TFM, nothing is written to tape.

    The find(1) instruction on its own is OK, as

  find / -local ! -type d -mtime -7 -type f -print > backuplist

works.  But then

  cat backuplist | tar ca-

still doesn't work, even though backuplist has hundreds of entries.

    What am I doing wrong?  All advice appreciated...

    Hugh Cartwright.  Physical Chemistry,  Oxford.